a while back a couple of other people had the same problem as you.
i can't find the thread though.
The topic is a little sticky since it's your father.
He'd probably be mortified if he knew you found out.
I don't suggest approaching him.
one thing someone suggested last time was to leave the browser open to YWN, or another website with GYE's ad.
he may find GYE himself!
but really, i don't think there's not much you can do besides daven, as INH mentioned.
I don't think you should be going into his email and deleting the p***. you might get tempted to look at it, or he might find you trying to delete it. but i am not a posek. we need R' Guard for that
on another note, that must have been really frustrating for you to find out that your father who (i'm assuming) you look up to and trust, is actually engaging in inappropriate internet activity.
are
you ok?