“I can’t take back your cup” what to say?

I’m a barista and today in drive thru, these customers sipped out of their drinks before wanting us to remake them because they didn’t taste right. That’s fine and everything but they put their drinks onto our drive thru countertop and when I told them I can’t take back drinks that were sipped out of and that they’d need to keep them, they refused and said they aren’t leaving them in their car and are instead going to take the fresh ones. I mean I get it but what do I even say? How do I enforce this policy? Do I just take the sipped drinks, dump them then wash my hands after and move on? I’m trying to see what you guys do/say because it feels weird to have a valid policy regarding food safety be so easily disrespected.