We've had problems using forward slashes in adverts for a looong time and I'm not sure there was ever a good way to 'fix' it.
Gmod sees // as comments. Again, it's Garry's way of adding comments. True lua = "--"
At the time, I think string.find and other string.replace commands would actually ignore the // because the engine saw it first and thought it was a comment.
It's possible that may have changed now.
I'd have to search 2 or 3 years back, but a discussion here on the forums with someone reporting the same adverts issue ended up being 'don't use //, use "Visit site
www.blah.com/pagename.blah" '
EDIT - Found this.
http://forums.ulyssesmod.net/index.php/topic,1260.msg5994.html#msg5994No evidence in that post that we tried fixing it, but I seem to remember discussing it with Megiddo in Steam IM.
Again, it's possible we could find a work around now.