Good Day Duncan

Please note that there is no way of blocking a user from a specific GL account, they either have access to GL or no access at all.

You will need to go to setup... users/passwords ...users...select the user in question and click on the Menu Access tab for that user on the right-hand side.
Click on the View Menu... and deselect General ledger, or else deselect only what they should not have access to under the GL access. Please make sure that you save the changes by clicking on OK.

Kind Regards. 

Ok noted. Thank you for the clarification and instructions.