I've thought this over, and I think I understand.
Recently some sites have started to open an all page advertisement at the bottom of page. It looks something like a second webpage overlapping the first, at the bottom of the screen. There is no (x) to close it, and it insists on getting in the way when you try to scroll the site. It almost always says Google in the title bar.
My method for dealing with it is to leave the site. If they are going to annoy me with adds and features that prevent me from reading their content, then they can go without my page views. If they do it consistently, I'll complain and leave permanently until someone comes to their senses.
At any rate, it's a website issue. There is little you can do other than complain. It is possible that the WebBlock and/or AddBlock apps might fix this, if they are indeed being sent by an add server.
On the other hand if you are talking about tabs being at the bottom of the page, I have no idea. This should not be possible in Safari. Well, unless you jailbreak, but that's a whole other game.