[Kameron8]

If you unequip spell thief as a class, you do not lost access to your stolen spells. You can cast anything you had previously stolen as any other classes.

[Exxy]

I've been told this only applies if you remain in the Rogue Class Tree (not saying this is fact because I have no way to verify at the time of posting this).

--> Either way this needs to be fixed ASAP; a suggestion Chaos made in another post was for them to go "Bye Bye" when you un-equip the class (which I could 200% approve of to punish Omni-Classing; a mechanic I whole-heartedly believe should be in the game but that's a bit off-topic).

[Neus]

This has been corrected in 1.57b.