You want to lock or unlock transaction codes in your SAP system.
Transaction code SM01 is used for the control of transaction code. From the SM01, you can either lock and unlock transaction codes. This SM01 is not advisable to execute because it can cause inconsistency in users administration.
1. To lock a transaction code (for example here, ST03) put the transaction code and then press Enter key which lists it in screen. Tick the box of the transaction and chose the button Lock/Unlock to toggle as required.