Upload & Sell: Off
Start > All Programs > Accessories > Snipping Tool
As mentioned, if you can see it, it can be grabbed...Windows 7 (maybe Vista too) gives you the tools to do it.
If your watermark is only across the bottom, then it doesn't require PS to remove it, but just a simple crop.
Place your watermark across the center of the photo, or in multiple places. Yes, you will get a couple complaints that it goes across their face, but it shouldn't be massive. This still won't stop people from publishing your (my) photos in an event program or even the Yellow Pages (yes, I'm serious).
But with the watermark over more of the photo, it will keep 99% of the people honest. The other 1% you can not do anything about, they won't ever pay anything and will either try to get rid of the watermark, or simply don't care about it. Again, to thwart this 1%, you have to sacrifice the other 99% and not post online at all.