I opened a support ticket and got this advice which does have a positive effect on the issue: reduce the history days saved from the default of 20. I chose 2 days and let it run for more than a few days. Each SEUM-User-# object uses about half to two-thirds than previously and appears to be stable. I'd call this a solution as it appears long lasting and doesn't require scheduled tasks or scripted processes to run.
On a side note, these instances are running IE10. To maintain compatibility with the monitored websites at least one of the three players we've spun up has been downgraded to IE8, which may open a completely different can of worms. Or not. We'll see and I'll post my findings.