Stealing from an allotment is a really low thing to do, and makes everybody feel uncomfortable. We have had similar problems this year with even, rows of potatoes dug up. I think in these cases the police should be informed.
If there is proof that another plotholder has done the dirty deed I can't see how they could stay on as plotholders. Our committee has a policy for practically everything and in a complaints and dispute policy that is covered in dismissal from the site. We also have a members responsibilities leaflet in which it is also covered. The difficulty comes with proving it unfortunately. busy_lizzie