Sadly, there is little you can do, regardless of how frustrating this may be.
From the experience of some of my soc members,
MA will not release you from your discipleship, even if you lodge a support case.
However, if your mentor does not release you within a reasonable period after you reach 'graduation' level, you can then contact support and ask them to finalise the discipleship for you.
You might also want to consider joining a society that helps new players - regardless of whether they have a mentor or not.
Perhaps the 'reputation system' that Marco has referred to will eventually reduce this sort of thing continuing to happen.
It reflects poorly on the game, and discourages new players from continuing to participate.