[smabie]

Just end the interview early: why waste their time and yours?

It's similar to making a decision to fire someone: if you have a suspicion that someone won't work out, probably best to just immediately cut them. Don't think I've ever seen anyone in my entire career come back from poor performance unless it's due to unique / temporary life circumstances.

[cbreezyyall]

This doesn't really seem like an option; normally I'm just one of a series of interviews and I can't just hand them off to the next person.