I don't believe you can, I'm pretty sure it saves whatever you have copied until you replace it with something else. You could copy a blank space I suppose
Just copy a blank space or anything else you want to overwrite with. Also, turning the phone off and on will erase the clipboard.
Edit: Yeah, the poster below is right, you can't erase the CB by turning off. I just threw that out there, since I figured it would, but should've tested. Sorry!
Yea, would be nice to be able to purge it. Might have to see if i cant make a little tool that does that, I really hate the 'Paste' option comming up all the time when i really only need the clipboard for a moment...