did you make them an assistant or secretary? If so, they can do that. If not, make sure the permissions for the plots are not set so that they can break and create blocks. I think the command to do that is /plot set perm reset or /town set perm..<I cant go past this>. You can do /towny for all the commands. I apparently cannot see some of the commands because I am not in a town haha!
Im not an expert on these things, but this is what I would try first. Im not sure if rules have changed for towny in the past 4 months. Hope this helps!