Provided they're not part of the town, they can't edit anything, which means all of your constructions are perfectly safe - staying inside your own home would be enough to keep them away, with some creative door construction.
As CJ said, they're not doing anything that's actually wrong - no griefing, PVPing in a non-PVP area, nothing like that, so there's not much to be done. Post a report if they engage in an actual legitimate activity.
As far as the lava is concerned, asking a mod about it next time you see one should suffice to have it removed.
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