In considering memory, it is helpful to begin by making a distinction between retention and recall. Retention is the ability of the mind to take in and store information while recall is the ability to bring a particular piece of information back to conscious awareness. There is considerable evidence that our retention is much better than our normal recall would lead us to expect - indeed we may retain all of our experience. For example, under hypnosis, people regularly recall whole chunks of their past with considerable detail. Likewise brain stimulation experiments using tiny electrodes have enabled people to vividly relive random previous experiences with great accuracy.
Normal recall is aided by two main factors - uniqueness and associations.
Likewise, if you can associate a picture, a sound, an idea, or a smell with any of those syllables, that too will make it easier to recall. The importance of both uniqueness and associations is that they allow a piece of information to be more easily distinguished from the mass of potential memory. If the emotion is positive, the stronger the emotion, the easier the recall, but if the emotion is negative, the relationship is not so simple. Your mind, however, can't do this by simply preventing recall of only that memory. The problem is that such a precise block would be like a silhouette - its precision would give away the memory it was attempting to hide.