Which version of Monarch do you have?
Is there no alternative trapping option that allows you exclude subtotal lines?
If not the usual approach is often to map a field in the part of the line that will contain the character string that you do not want (in this case) and then filter out any lines that show the identifying string via a filter.in the table.
There are some variations on that theme for identifying the line but the filter in table part is usually the option.
If you have V14 you could look at some sort of Regex trap solution perhaps but usually there will be some way to make the trap work for you related to line formatting rather than specific character string identification.
If you could post and example of a wanted and an unwanted line we could have a look at the options to see if there is something you could try without going for complexity.