There are 2 very simple ways to restrict the customers who can utilize particular payment methods at checkout:
By Customer Type
Within each Payment Method there is a dropdown menu to select:
Note for the system default 'Charge to credit account' method, this is locked to accounts with Credit Account enabled.
All Customers (anyone using the store can utilize this Payment Method)
Account Customers (only users or accounts with Credit Account enabled can utilize this Payment Method)
Credit Card Customers (only users or accounts with Credit Account disabled can utilize this Payment Method)
By Customer Label
You can restrict a particular payment to customers that have a label assigned to them, so that it is not visible to any other customers.
You can catch up on our 'Labels' article by clicking here
Select the required Payment Method and click the 'Restrict By Label' option, in the label field either type the required label and select or press the space bar for a list > Save Payment Method to complete.