Super powered RP. Player created a character who could change their mass to any degree, becoming super armoured, super strong, shoot projectiles made out of bones and grow to the size of giants and retain their strength.

Claimed they wouldn't use it because their character was too young to understand it properly (All characters in the RP haven't had their powers for long, but none so blatantly powerful). 

I pointed out that if the powers weren't going to be used there was no point having them... after some back and forth they're no longer taking part.