It only works if you let it get to you, by doing so you give her/ them power over you. Take the power back, ignore them, act as if it doesn't matter because, in the end, it doesn't really matter. Focus on the people who care about you, not the half witts who have nothing better to do. :)
Bullying on mibba is not acceptable and you should report the person. You shouldn't leave mibba because of that person but you should avoid them. You should change your username if you don't want that person to keep finding you.