Don't answer it - let it go to voicemail. Then, when you finish the current call, handle it from VM - call back, ignore it, whatever.
On an outgoing call, you can dial *70 in from of the number to stop any incoming calls, but you can't do that if you're on an incoming call. (And that's up to your carrier - so check with them [not the MVNO if you aren't using a carrier directly] to see if they have any "stop call-waiting after receiving a call" function.)