Im a newb at this too, and really disappointed with the lack of response one gets when asking for help here, so I'll take a bash at it: Look for the form thats used when viewing or editing the user profile. I believe its register.asp You probably can just comment out the URL part of the form, or you can force it to be set to nothing. About halfway down in register.asp you will find the following line:
If strHomepage = "http://" Then strHomepage = ""
endif
I would change that to simply
strHomepage = ""
and it should force every new user to have a blank homepage. You would have to manually change the existing ones who have already set it to something else.
Keep a copy of the original register.asp, so you can always revert back to it in case you bonk it by following my suggestion here. Test the change, your mileage may vary, professional typist on a closed keyboard etc etc