That's an odd thing, and put some odd ideas in my head.
Most of the time, if you are trying not to think of purple elephants, you can't help but think of purple elephants. The mind doesn't process negatives like that as well.
But in a world where telepathy is a real thing and should be defended against, it's probably possible to learn to do the mental gymnastics to distract yourself on cue and think of something else. It would be easier if you had a telepath to help train you, since he could gauge how well you are doing.
One method would be total focus on what you are currently doing, thus pushing the secret password out of your mind. Another might be the annoyingly catchy song technique, which would have the same effect, and also might allow you to get the annoying song stuck in the telepath's head as well.

Obviously, a psychic using Mind Block powers would be much more effective than this. And these techniques wouldn't require any new rules. They'd just be flavor for those O.C.C.s that some disciplined soldier or spy types have that allow them to defend against mind reading.
And of course, you can get a little mean with this. If you have one of those obnoxious characters who gets super paranoid and says "He might be reading my mind! I shift mental focus away from the secret password!!" 50 times a game session, for several months, then it would be perfectly acceptable, when it comes time, to say, "Um, well, for the past six in-game months, you have been continuously trying not to think of the password. You have to make a roll to actually remember what it is now."
That, to me would, make perfect sense. If I read your mind, and you imagined a brick wall with the password behind it, I'd see the brick wall, and the password behind it. You'd have to deliberately not think of it, shove it out of your head, in fact, temporarily "forget" the password.
You can do that a few times. If all you do is not think about it, and put it out of your mind, then you aren't really trying to remember it. Quite the opposite, in fact.
All I need is a warm bed, a kind word and unlimited power.