Blocking a buyer

[nananspider]

I would like to block a buyer from making purchases, when I type in the user name then submit it’s stating error 

what am I not doing? 
thank you in anticipation for your help 

[the-nutwood-collection]

If you add the User ID to the end of the list (rather than inserting it in alphabetical order), put a comma after it.  When you submit the form the app will re-arrange the User IDs into alphabetical order and, if the new ID doesn't include a comma, the form will be rejected.

[tobiasd4]

You have to put a comma (,) after previous buyers id to separate the names.