We are doing some work on our garden and hired a skip so we could get rid of an old shed and some garden debris. All week it's been fine, no one had put anything in and I was very much of the "not in my area" mindset.
Until yesterday. We had found a bag of open dog shit in our skip and had suspicions on who it was. We set up a camera and waited and caught the guy in the act. Went and put it on his doorstep as I know where he lives (he walks his dogs regularly past our house) thinking he might take the hint.
Saw him go past this morning so watched for him coming back at our hidden side window only for him to do it again! So I confronted him about it and he denied it outright until I got the video up on my phone. Said I would keep putting the shit right back on his doorstep if I saw him on camera again.
Woke up this morning to see someone had dumped a huge plastic pot of tile grout in there. Checked the camera and it's the neighbour a few doors down. Took it down and stuck it right in the middle of their driveway. He suddenly appears yelling "You can't dump your shit in my garden". Tell him we have him on CCTV dumping it in our skip and I will happily take it back for 10 quid. He is livid and offended that I would even suggest that he would dump it in my skip right up until I show him the video 
I now have sign up telling people not to dump stuff and that they are being recorded, but AIBU to dump peoples shit right back if I catch them on camera doing this? I paid 200 quid so I could clear my own garden, not so I could fund a giant community bin! Sorely tempted to spam them on social media too (I won't of course).