I had one, lovely couple, a bit OCD.
Used to wash up with bleach.
Tea/coffee/squash... It all tasted like bleach.
I now rock up, hop out if the van then while they're looking take a swig of coke from my van.
Then when they asked, I'd say, I've just had a drink, thanks anyway.
They stopped asking after about a year of me doing this.
Lovely couple-I just couldn't face the bleach and didn't want to hurt their feelings.