can you not also delete the security access cards (if any?).

oic.
ok, if cut water and electricity, they also can re-connect back and if the police also cannot help, i would resort to drastic measures. I would go downstairs and seek help from the downstairs neighbour. Ask them to allow you access to their bathroom(s). Unscrew the pipe and stuff a cloth in it. It will create a backflow and flood your tenant's bathroom(s). Of course, if they ever go, you will need money to fix the bathroom up again. But then again, this is only for drastic measures. A suggestion albeit all legid measures have not been able to help solve the issue.

good idea!