You can't clear those! They're there forever and ever. Everyone who sees your computer (and also the C.I.A.) will know you were looking at gross porn.
Just kidding. There are a LOT of things you have to clear. First, you have to clear your computer's "MRU" lists (Most Recently Used). A good spyware/adware scan will take care of that. (If you don't have one, Google "adaware" - it's an excellent free one, but does require some setup.)
Next, try clearing your Internet browser's cache, cookies, history, etc. (Open your browser, select "tools," and then "options" and it should all be in there.)
If those things don't take care of it, you can try rebooting and see if they're gone.
If all else fails, use the "help" function of the program that is showing your porn videos to try to figure out what to do.
And remember ... porn sites have lots of spyware, adware, and even viruses.
;)