What is the term for "What has been seen cannot be unseen" in cognitive sciences?
The most relevant term would be Ironic Processing.


  Ironic processing is the psychological process whereby an individual's deliberate attempts to suppress or avoid certain thoughts (thought suppression) render those thoughts more persistent.


This is a result of a failure of Thought Suppression or more generally Motivated Forgetting. It appears Thought Suppression doesn't work well and generally the attempt to not think about X results in a sudden increase of thoughts about X. 

Since you have a sudden increase of thoughts about the "suppressed" thought, you're strengthening retrieval of it, and you might even be conditioning that thought with the reason you don't want to remember it.