Hi Atto
Please do a trace and check the TAC number of the phones used by those IMSI.
If they use ZTE phones (A35 for example) then you have the same problem as some networks are facing now:
If a deleted Sim is inserted in some ZTE phones, it will send LU requests every approx 2 sec unless the person itself takes out the deleted sim from the phone or unless you create again the deleted sim and call the guy to stop.
Cheers