LOL The horrible rag “Olympian” is doing the same thing at my house and I have to clean up the mess every day.
awww...I'm very sorry to hear that they are doing this to you :-(
"Wondering if it's feasible to collect them for several weeks, then put them all in a big box and return them 'freight collect'?"
You're only returning their property, after all, which they apparently left at your address by mistake.
((((evil furry stoat-wink)))))