1. Click 'Availbility' from the Status menu
2. Click on the account listed at the botton that you want to disable (e.g. jabber)
3. Scroll to the bottom of the Edit Account page.
4. Click 'Enabled' - i.e. make sure there is no check mark next to enabled.
5. Click Save
6. Click Save again
That does not work. The online status and modes apply to all active IM accounts at once.
You can edit a status (new one) to switch the statusof each account to a specified value. I use this on a new status 'Calls Only' to allow only Skype and SIP accounts, leaving the others busy or offline.