Goto ACP > Configuration > Showthread Options > Postbit Layout and choose the desired postbit style. This will set the default but to change existing users you will need to run a SQL query via phpadmin.
For classic postbit:
Code:
UPDATE `mybb_users` SET `classicpostbit` = '1' WHERE `classicpostbit` = '0'
For horizontal postbit:
Code:
UPDATE `mybb_users` SET `classicpostbit` = '0' WHERE `classicpostbit` = '1'
(replace mybb_users with the table prefix if you changed itand you should have)
To remove the option for members to change it from UserCP: Goto ACP >Templates & Style >Templates >your current theme > User Control Panel Templates > usercp_options and search for the following code: