Hi Raj,
You cannot deactivate/delete MRU's if it is already assigned to Installation/s or if schedule records exists for that MRU.
Regarding 'activation' of MRU, if you have performed any changes in the Portion you can activate these changes on all the MRUs that have this portion assigned. This can be done using E43E tcode.
Regarding restriction on assignment of existing MRUs to new Installations, please check if this can be achieved by specifying the Billing class and Division category restriction on MRU (TE422AS table).
Regards,
Mayuresh