I'm going to ruin a first date
Why do you want to ruin a first date?
Because the guy is really boring, he talks all the time about himself without giving me a chance to speak.
It is realistic that you are going to have some first dates that do not go well. The entire point of having a first date is to find out if you think the person is worth getting to know better. It is more moral for you to finish the date however rather than making the date terrible on purpose.
You are a liar.
I'm just saying that there isn't a need to ruin the date. Just don't call this person again if you don't like him.