"I DON'T LOVE YOU"
"I'M SEEING SOMEONE ELSE - I'M GAY!"
"I FANCY YOUR MOTHER MORE THAN I FANCY YOU"
"YOU ARE UGLY - GO AWAY!"
"GO BOIL YOUR OWN BUNNY"

Failing that you could change your mobile number, move to another town..... or just say "Look fuck off, I cannot stand you, you are the most annoying bint I have ever met, I do not love you, you don't do anything for me.... "

I'm running out of ideas now......