Go to Gear Icon/Manage Users and click tab for Accountants. You should see the firms and you can delete ones you don't want to have access anymore.**
** This does not apply if they are listed as a Master Admin or your billing is being handled by them (Wholesale Billing). If that's the case, the firm needs to remove you from their wholesale billing account and transfer the Master Admin rights to another user and then you can remove them as an accounting firm.