UIM 9.X and earlier
robot 7.95 and earlier
hub 7.95 and earlier
There is no way currently at the hub application level to prevent this.
There are two options for this
1) deactivate or uninstall the robot service
2) set up a firewall rule blocking port 48002 coming from the robot you wish to remove.
Once one of the above is done you then can remove the robot from the hub robots screen and it will not return.