Yes, a supervisor can be demoted for abuse of power.
If a WikiAnswers Supervisor is found to be abusing any of their powers, they are demoted immediately and taught how to use them correctly. If a Supervisor is knowingly abusing their powers, they are blocked from contributing.
A supervisor who is stealing answers from the other users can be reported and he will be banned or demoted by the site.
The user can be a supervisor as long as he/she wants to be, or he/she is demoted.
A Supervisor can be a Supervisor for however long he or she wants until the Supervisor deletes his/her account, is blocked or demoted for violating rules, or wants to quit being a Supervisor. If a supervisor remains inactive for a certain amount of time, he/she may have their supervisor powers taken away until he/she contributes again.
Hit the report abuse button or report them to a supervisor
You can report a supervisor who is abusing his power to the website.
Supervisors are expected to behave by the same guidelines and TOS as normal users. If one is found and charged with breaking the rules they may be demoted back down to a normal user and depending on the offense they could have their account suspended as well.
The manager has more power than the supervisor because the supervisor reports directly to the manager.
When people abuse the power they have, it is commonly referred to as abuse of power or power abuse.
It is NEVER illegal to petition in any case according to the first amendment of the Constitution of America.
Supervisors cannot be blocked though they can be demoted and then blocked. For this reason, it is impossible for a supervisor to block themselves.
See the related question - to an excellent answer by a fellow supervisor, as to what happens about reporting abuse.