I was asked out on a date but I told him I couldn't, as I'm in a serious relationship.
Does he know that you were not joking?
Yes, but he didn't care and he said I was a jerk to him.
The polite way to decline a date is to thank the person for asking and tell them why you're not interested while still being mindful of their feelings. If he can't understand it after that, then it's time to remove yourself from the toxic situation.
I'm tired of people not respecting me and the things I tell them.
Your frustration is understandable, but there's not much more you can do. You did the respectful thing to remain faithful in your relationship and you were honest with this person. You were valid in your reasoning and it's his problem if he can't respect that.