To remove a system from Promapp, first you must remove system tags by going into each process and removing them manually. This means from each activity, task, and note.
If you have a number of processes where a system appears in, this is a significant amount of work, before you can remove something that may be obsolete, or incorrect.
I suggest, as each system has an owner, a notification for system removal would go to them, and they would review and delete the system, if applicable.