That's a difficult 1.
Normally, if a memory isn't thought about for a long time, it gets forgotten; although "forgotten" is an ill-used term. It does not mean that something completely disappears from memory (although brain trauma can cause that to happen), merely that it becomes harder to recall.
I differentiate between memory & recall. It's often said that so-and-so has a good memory. In truth, what we should say is that so-and-so demonstrates above-average powers of recall. I think this difference must be noted as without it there could be no such thing as suppressed memories.
However, I should actually say "so-and-so demonstrates above-average powers of recall to his conscious mind" as memories can affect our subconscious also. That is when suppressed memories can become a problem. If a memory is locked away and never rears its ugly head, it can do no damage. But if that memory doesn't come to the conscious mind, merely tickles the subconscious, it can cause strange and, seemingly, inexplicable effects.
How a memory can be locked safely away is something I cannot answer. Not thinking about it will simply push it further from the conscious mind but will not necessarily prevent it playing havoc with the subconscious. Potentially dangerous memories are often better brought to the surface so that they can be handled consciously. However, techniques for doing that are sometimes a bit dubious and the possibility of False Memory Syndrome must be considered.