We occasionally have a waste skip on our site to dispose of items like plastic, broken glass, old pallet wood (bonfires are banned on our site) and carpet which has been used to cover plots (that's now banned here as well) and we constantly remind people that when we have skips they are for items from plots only. But every time we get plot holders bringing in items from home to put in the skip (unseen), as they have done with the skip currently on site. They have to load items into their vehicles so why can't they take them to the local dump, they don't have to pay to dispose of items there. We've got a large number of dead Box hedging plants, lots of what was very tall Privet and lots of branches from a climbing rose plus a large amount of broken up concrete (in bags), and some old partly broken up doors - probably almost half the skip is of these non allotment items and because its now full plot holders who have 'genuine' items from their plots can't put them in the skip.
We have often thought to do the same and put it to a vote at committee meetings but we always vote it down. Skips are expensive here in London and we take the view that your rubbish is yours to clear from the site and we just know that someone will take the mickey..
However I recently got a skip for the house to clear the garage and some building work debris. So much stuff got taken off the skip by passers by that we told our neighbours to bung stuff in if they wanted!
had fly tipping on our site, so set up a wildlife camera and caught the culprit who was also a plot holder (now no longer)
When we had free skips from the council we had the same problem...... I ended up skip monitor...........