Although this won’t solve the problem generally, this is something worth pointing out to the individual course maintainers to fix. What they can do for words that have other meanings is to create an extra column in their database named something like “other meaning”. The real answer goes in the English column, but any other meanings go in the extra column. That way, when the user enters one of the other meanings, Memrise will tell them “you typed the Other Meaning when we wanted the English”. You don’t get marked wrong or right when this happens, you just get prompted to answer again.
I use this in my Hebrew course. For example, I want people to learn different meanings of the world שָׁלוֹם, so I have two separate entries in my database for it. One entry has “hello, goodbye” in the English/answer column, and “peace” in the Other Meanings column. The other entry has “peace” in the English/answer column, and “hello, goodbye” in the Other Meanings column. So if you’re reviewing and you see שָׁלוֹם and type “hello” but you’re actually being asked to review the “peace” entry, it tells you “you typed the Other Meaning when we wanted the English”, and you’re not marked wrong, you just know you have to remember one of the other meanings of שָׁלוֹם.
My example is two entries in the same course, but it could easily be used even if the course only had one of those entries. It simply needs to be aware that the word it’s trying to teach you one meaning of, also has a different meaning that you might happen to know even though it’s not in the course. It could put that in the extra column, just in case someone taking the course happens to learn it from somewhere else, so as to avoid marking them wrong if they mistakenly enter it for a review in this course.
So, while your suggestion for Memrise is a good one, I think you should also post in the applicable language topic for each of the courses where you encounter this, tag the course maintainer, and ask them to add this hack to their course.