To lock, unlock client, use transaction se37
For this operation use functional modules:
1. SCCR_LOCK_CLIENT (to lock the client)
2. SCCR_UNLOCK_CLIENT (to unlock the client)
This function is used to lock \ unlock client from logins. When you lock a client, all users try to log on A client will show message “A client currently blocked from entering.” A client will be available for entry only available to users SAP * and DDIC.
1. Run tr. SE37
2. Enter the name of the module SCCR_UNLOCK_CLIENT
3. Press F8 to activate
4. Enter the number of client and press (F8).
To lock the procedure is the same, only with the module SCCR_LOCK_CLIENT
Reference: Blog SAP Basis Administration