If you're worried about mess, don't feel that you have to carry on if you see, or smell, anything.
I always just stop if that happens and quickly deal with it as necessary then do something else with them instead. I've never had anyone complain or try to insist that I carry on regardless.
Also a good idea to use a condom on the strap-on or toy so you can withdraw slowly and roll the condom off as you're pulling out so if there's a bit of mess it will be safely encapsulated inside the now inside-out condom for disposal.
Sorry for the graphic details.
I've tried explaining about douching before anal sex to multiple regulars but rarely manage to convince anyone to adopt the practice. If anyone knows of a way to talk them into it, I'd love to hear it.