There are many ways, but this one is elegant in its simplicity. It may be more fun to find one niggling detail, (detail unimportant, in fact, the pettier the better), grip it tightly in bulldog jaws and keep pulling at the idealist, but I think this one is better. Less is more.
Do nothing, say nothing. No matter what the idealist says or accomplishes, remain quiet.
This way the idealist is left to wonder if anybody is home, and the sneaking feeling that nobody’s home will crush that sucker faster than a steamroller flattening the coyote in a Roadrunner cartoon.