The old saying, "What you resist, persists" is now borne out by research:
Mind - Why the Imp in Your Brain Gets Out - NYTimes.com
This is also evidence of the NLP dictum to only speak in the positive. The usual example is "don't think of a purple elephant!" What happens for you? You think of a purple elephant, because you have to make the picture to get rid of it.
If I say, think of a red rhinoceros, what happens? You think of a red rhino, not a purple elephant. So if you want someone not to think of something, direct them to something else. The easy way to accomplish this is, eliminate the word 'not' from your vocabulary.