Easy fix providing you're on PVE and its a claimable lot - wait till its quiet - pull your flag down and put it back up. You will have full control of all crap on your claim. Be aware if you have a water claim with land claims nested inside it you need to remove water claim first - then pull land claim. Put back in reverse order - land claim first, then water claim.
SIDE NOTE: Go into your COMPANY settings - next to the CREW LOG is GOVERNANCE tab. AT the bottom under DEFAULT TERRITORY SETTING - change this to ONLY COMPANY CAN BUILD. Next time you claim land it wont default to EVERYONE CAN BUILD.