To Disable multiple logins in the same client,  implement the parameter in the instance profile.
login/disable_multiple_gui_login = 1

If you do not use this parameter in your system, users have the ability to ignore this warning window at the time they try to login to the same client. Activating this parameter in your system will make you look good incase you are audited.Now, what if you want to allow logins for some users but not all. There is a workaround for that too.
Implement the parameter login/multi_login_users
You can list the users ids that should be ignored if the first parameter is active.

If you like this post, you may as well like these too:

1. What is a Client? A client is a independent unit in the SAP system from an organizational point of view. Each client has it’s own data environment. The client concept allows for the...
2. Disable “Import All” in STMS Queue Login to your Transport Domain Controller. Run STMS –> Overview –> System Choose the system you want to disable “Import All” button. Goto transport tool tab. Add/Create Parameter “NO_IMPORT_ALL”...
3. Installation of Multiple SAP Systems in MSCS: Oracle How to Install multiple SAP Systems. Download from Rapidshare or Mediafire. Contents Chapter 1 Introduction Chapter 2 System Configuration in MSCS 2.1 SAP System Components in an MSCS Configuration...
4. Maintain Multiple Shift Depreciation The following steps are needed to maintain multiple shift depreciation: The variable depreciation portion as a percentage rate is to be maintained in the detail screen of the depreciation...
5. Assing Multiple G/L Accounts in Purchase Order We can assign multiple G/L accounts in the Purchase order for the same line item. The costs can be allocated on a percentage or quantity basis. If the partial...