All a Russian would have to do is buy the service under a phony Ukrainian identity with mobile service. As long as the service is available in Ukraine it is likely impossible to stop a bad guy from using it.
Except that our own Dept of Defense is in control of Starlink and everything that goes through it. If the enemy is using it they would absolutely know and have the option to immediately shut that device down or continue to allow it to be used by the enemy.
There is no way Russia could use it for military purposes without them knowing. So if it is becoming a problem then our own DoD and/or intelligence agencies are allowing them to do it.