One word: army.

Pick some well organised enemy that, in a group of 20 or so, would be a slightly challenging encounter. Then have 5000 of them show up.

If the party insists on fighting, it might be hard not to kill them. But if the enemy surrounds the party and doesn't attack, that should send a clear enough signal as to what's up. If they insist on fighting anyway, you're going to have to think of a good reason for not killing them.