You can remove entities on plots you do not own if there is at least one cactus on the plot. First need find a plot with entities and a cactus, then go up to the entity on the plot (e.g minecart) and start pushing it towards the cactus (to push the minecart you may need to crouch). When the entity hits the cactus it will either damage or potentially kill it, or in the case of a minecart it will break it and drop it. I found this when I was playing on someones plot where they have a cactus and a few minecarts on the floor. It can also work for any entity or animal.