Look, I know it’s a bit immature, but sometimes when you know you’re right, all you want is to get the last word and shut down whatever debate you’re in for good.

If you want to make it clear that your word is final, you’ll have to choose your words carefully. Here are some things you can say to put a full stop at the end of an argument right away.
1. “I appreciate your perspective, but I’m going to stick with my own.”

If you want to disagree politely without getting into a lengthy debate (or shut off a debate that’s already gone too long), this is an option. You’re validating their POV but letting them know that yours trumps theirs in this case.
2. “We’re clearly not seeing eye-to-eye on this, so let’s just agree to disagree.”

This is a classic way to gracefully exit a conversation that’s going nowhere. You can acknowledge a difference in opinions while also pointing out that it’s pointless to keep talking about things. In fact, this is a really mature option.
3. “I’ve said my piece. I’m not going to keep repeating myself.”

This is a great way to set a boundary, and it also prevents you from getting dragged into a circular argument. It shows that you’ve expressed your thoughts clearly and aren’t willing to waste time on further repetition. It also conveys a sense of confidence and self-respect.
4. “I think we’re just going around in circles here. Let’s take a break and come back to this later, maybe?”

If you need a way to de-escalate a heated or unproductive discussion, this is a good option. You’re clearly getting nowhere, so why shouldn’t you take a pause so you can both calm down, think about things, and potentially come back to the conversation later (or never)?
5. “I understand you feel strongly about this, but I’m not convinced.”

You can validate someone’s feelings while letting them know that you’re not on the same page. It’s respectful to hold your ground while not totally eviscerating their clearly wrong perspective.
6. “I’m not interested in arguing anymore. Let’s focus on finding a solution.”

At a certain point, if you’re not making progress on seeing a problem the same way, you need to just shift your energy onto finding a way to solve it. You don’t really have to understand each other to come to a solution that you’re both comfortable with.
7. “I hear what you’re saying, but I have a different take on it.”

Again, you’re not invalidating them, but you’re letting them know that continually repeating their perspective isn’t going to change yours. It’s as simple as that.
8. “I’m not comfortable with the direction this conversation is going. Let’s change the subject.”

You’re willing to hear them out and have been doing so, but if they’re becoming disrespectful, overstepping your boundaries, or badgering you to say things their way, cut them off and put an end to the conversation, end of story.
9. “I’m not going to engage in a shouting match. If you want to talk calmly, I’m here.”

Exactly what it says on the package — if they want to scream, belittle you, or act like a maniac, you’ll be tuning out. They can get themselves together and come back to you when they can talk like a normal person.
10. “I think we need some space to cool down before we continue this conversation.”

When emotions are running high, sometimes you just need to shut down the conversation before either of you says or does things you end up regretting. Saying this shows you’re actually being the bigger person!
11. “I’m sorry, but I’m not going to let you disrespect me like that.”

Again, don’t tolerate disrespect under any circumstances. People can make their thoughts, feelings, and opinions clear without treating you like rubbish, so they’ll need to do that if they expect you to listen.
12. “I’m choosing to walk away from this conversation. It’s not productive anymore.”

This signals the end of the discussion, and sometimes that’s what’s necessary. It shows that you’re not willing to waste any more time on an unproductive or harmful argument and are putting yourself first.
13. “I’m not going to change your mind, and you’re not going to change mine. Let’s leave it at that.”

You’re agreeing to disagree here, in essence. There’s no sense wasting either of your time or energy continually repeating your position when the other person isn’t going to budge. Just put yourselves out of your misery, already!
14. “I’m not going to apologise for having a different opinion.”

And why should you? You’re as entitled to your POV as they are to theirs, and if they say differently, they need to grow up.
15. “I’m going to take some time to process this. We can talk about it later when we’re both calmer.”

Again, this is very mature (and perhaps very demure, very mindful — sorry, I had to!). If you feel your blood pressure shooting through the roof because you’re so angry, you’re better off stepping away for a bit and coming back to the discussion when you can speak more calmly.
16. “I value our relationship, but I won’t tolerate being spoken to like this.”

No matter how much you like someone, there’s no excuse for them talking to you like you’re scum on their shoe. If they can’t show you the respect you deserve, they don’t deserve to speak to you.