From what I have heard, "locked plot" has something to do with townyadmin, I am not exactly sure what a townyadmin  is for but moderators are not able to do anything about it. Many towns have faced this problem and mayors claim they have not typed in any unusual command. This is a job for an admin, as for help with towny commands, you can either type /town help or look in the guide section of the forum and click on the very first link in red at the top of the page, it says, "Creating a town made simpler."