Lolz, this is sad...

How to stop your BF from being too close with a girl.
a) Buy BF cheating Insurance
b) Call Police to Lock the BF in Jail
c) Call Police to Lock the Girl in Jail
d) Find a Boy to marry the Girl
e) Leave them alone and trust your BF
f) Tell your BF your worries

The answer is sooo obvious when this question is asked. It's only the fact that someone is being very insecure.

How about just leave this BF and find another one who's GAY or do not have any female friend. (This is the only way to solve the problem if that's what "you" want)