Any time I think "in order for my game to work, the PCs must do this" I take a step back and make sure it's what I want to do. Sometimes the answer is still yes, but I try not to assume players will do the obvious thing, or that it will be fun for them. Some players would tell you that they'd rather die than surrender. I don't think that's realistic but I've had it happen.
So with that caution out of the way, why do you think RAW won't allow them to get hurt? Throw out the challenge guidelines for the encounter and send in a triple deadly force. You're the DM you can always throw more at the players than they can handle.
If they decide to fight anyway, figure out some way that you can knock them out of commission without using deadly force. Don't want them escaping? Block teleports or just drop an anti-magic zone on the party. Or get creative. I once had the floor break and crumble underneath the PC's feet as rotting hands reached up and grabbed them, pulling them down into the ground. I had the grapple checks high enough (and enough hands) to pretty much guarantee success. One PC almost got away, so I had a contingency planned, but yes the plan was to pull everyone into the Shadowfell.
But seriously. Expect at least someone in the party when faced with a "Surrender or die" scenario to choose the latter.