Android - How to stop a phone from ringing without rejecting the call?

I suspect this is very much device dependent, but on the HTC Desire HD, you can press the physical volume down button to silence the ringtone whilst allowing the call to continue.

The HTC Desire allows you to flip the phone face down to silence that call. If that doesn't work with the Samsung then you can probably use Tasker to arrive at a similar result (put the phone into silent mode when it's face down).

For Samsung Galaxy S, the way is to hold the volume down button. If you just press, nothing happens (or maybe you have to press it several times before it completely stops ringing, I don't know, but holding it makes it stop immediately)