Could I reject a girl as revenge?
Why are you taking revenge on her?
She has always disregarded me in the past.
There is no reason to take revenge on someone in that way. Just move on from the situation entirely.
I don't give a shit. I'm going to reject her like I always wanted to.
It would be petty and a shame to reject someone who you used to have feelings for that now has feelings for you. Why not try giving them a chance instead? You could both be happy for a long time instead of you getting a little bit of happiness for a few days at best.