Firmly place one, or both hands on top of her head. When you are about to blow, push her head down on your cock as hard as you can. That'll solve the issue.
Lsdies! And some guys too. IF you are SUCKING a cock, never be surprised when he cums. That is what he wants and why he has you sucking his cock. Goes for guys too. If you suck a cock you are going to get cum.