I had the exact same issue and can confirm it is fixed in the new version of Samsung Universal Print Driver 2 (2.50.5) . I was also running 2.50.2 and the key in question was using 275MB. Simply uninstalling the old version and installing the new version on the server (I did not update any client drivers) reduced the size of the key to 3MB. I also tried logging on and back off and can confirm it now deletes the keys when you log off, unlike before.
If you’re curious, before it would not delete the keys. In a single user environment this isn’t a problem but since the printers are named for the user and the session, it is an exponential problem (for a 20 user server like mine you generate up to 400 keys… and that’s if you only have 1 printer per client)
edit: the new key is called usp01 instead of spe__