i think that if it carries on i would have a quiet word with the allotment society our site is very laid back and they dont seem to have many rules but they do stipulate to respect other plotholders and if they're being a nuisance you should have reason to complain esp if they're damaging your plants, to be honest i go to the allotment for a bit of peace and quiet, i would be really hacked off if i had the hassle. good luck with it though!!