[Phalanx]

If she's being a sleaze, then I'd suggest treating her no different than you think a chick would treat a guy in this situation (maybe minus the slapping, hm?)
IF you've explained your point of view and wishes, in a manner not laden with innuendo, or any kind of double entendre...and you've honestly let it be known clearly that you are not interested...and it continues...I'd suggest something along the lines of -
"Ok, I got something important to say here...I've let you know this before but it seems I might not have made myself clear enough, so I really need you to hear me this time" pause. Make sure that much is understood...continue. "At this point, friendship is all I want for us, by my definition you're kind've messing with my boundaries. You should know that right now I have no intention of taking this any further than friendship" Pause...let it sink in a little..."This doesn't mean I don't like you and (if you do, of course) I enjoy hanging out with you. If I gave the wrong impression at first, I apologise, but it's important to me that as a FRIEND, you respect my wishes on this one."
and finally "now please, stop...I find it really hard to talk while you're sucking my penis."
See how I did that? All serious 'n shit, then with the levity...aaaanyways, the bit ABOVE the last statement should carry you through alright.
If you say all that, make sure you say it when you're not, AND she's not drunk...and if it continues, well, I say that obviously she isn't respecting you, and isn't at this point worthy of friendship...in which case, I guess you're open to being as much of a total cunt towards her as you like.