Is there a way to force AD garbage collector or deleting of the user

no

After the synchronization is completed, any accounts that were not set to active are permanently deleted from Unified CM when the garbage collection process runs. Garbage collection is a process that runs automatically at the fixed time of 3:15 AM, and it is not configurable. The deletion of Unified CM accounts that do not match LDAP directory accounts is necessary because Unified CM cannot manage accounts while synchronization is configured.

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/directry.html

HTH

java

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