In a way you find this function on the right of the input box where you enter the format string. in the shape of the arrow-to-the-right button.
At the bottom of the list of that context menu there are the function "Delete from list" and "Delete all". This should remove one or all unwanted strings from the history.
You have to do it one by one, though.
Or you create a little text file in your favourite directory where you store the best masks and then copy and paste them from there. This would also allow to add comments so that the masks are more easily to be recognized.
I do that for filters as their parameters are sometimes even more cryptic.