To lock or unlock a client only in an in R/3 System, run the following functions via tcode SE37:
1. SSCR_LOCK_CLIENT
2. SSCR_UNLOCK_CLIENT

Locking/unlocking the entire system, an admin can use the following commands:
1. tp locksys pf=
2. tp unlocksys pf=

- transport profile in /usr/sap/trans/bin usually -> TP_DOMAIN_.PFL
- SAP* id will still be able to login.