If they are relatively sensitive and observant (not a lot of chance with bodhran players!), they will often stop if you "throw" them - extend the pause between verses and come back in late, singing across their added rhythm. Extending a note to emphasise one of the words works, too. If they then carry on, you will undoubtedly get thrown yourself and have to stop - which should drop the message. Come straight back as soon as they stop.

I have seen one singer, having his free-time singing constrained by a guitarist's unwanted accompaniment, lean backwards against the guitar neck and mute the strings. The ripple of appreciation made the guitarist realise his mistake.

If someone realises their mistake in accompanying you, always thank them for their consideration in stopping playing. I generally make it my fault (I can't keep good enough time to stay with an accompaniment like that etc.).

The bloody-minded ones who will not shut up are nearly always the hopeless cases - incompetent and insensitive musicians with a hide like a rhinoceros! If it is obvious to the rest of the session that what they are doing is inappropriate, a stern remark will garner approval from the crowd and they generally won't retaliate.

....and then there are the noodlers, twiddling with their instruments while you are trying to start a song.....