N.B. The screenshots and descriptions are for the iPhone/iPad version.
Tap on Stables->Users and select the user from the list. Tap on the delete icon to the right of the list. That user will be removed from accessing your Stables.
Tap on Stables->Users and select the user from the list. Tap on the edit icon and if the 'Is Manager' switch is on, switch it off. Turn the 'View Vet and General Notes' switch off and save the record.
Tap on Stables->Users and select the user from the list. Don't assign them as a manager. You will then see a list of permissions for different actions which you can allow/deny them.
See also: Managing Users.