I hadn't heard of this site before, so I went to take a look.
Only 5 works at a time seems to preclude making much money - how much can people cough up to just look at something online on a device? And rentals to other members via subscriptions seems strange too, as does variable pricing as the remaining items in an "edition" dwindle - what's to stop anyone from licensing a similar on Shutterstock?
Anyway...
Here are the terms and conditions that say clearly you can't delete your artwork once "launched" and that they tot up what you're owed quarterly and pay you 30 days after that. You agree to leave your works there for three years from first launch and they have 60 days to remove it from sale. That doesn't affect continued use of previous sales which can continue.
https://www.seditionart.com/terms/artistNot sure that you have any basis for removal and has the quarter ended and the 30 days passed on the sales?
Hiring a lawyer is expensive, and unless you're owed a lot of money and these guys aren't following the terms of your agreement (as opposed to you don't like the terms), would you stand to gain more than the lawyer would cost you?
For anyone else interested in the artist info, here's their support page:
https://www.seditionart.com/support/artist