Is it possible to prevent agents from creating new accounts when creating a ticket? I would like them to be able to only select an existing account from our list of accounts.

We are having a problem where accounts are added multiple times as agents create tickets because they type in the name instead of selecting from the drop down list. Sometimes an account is entered as a duplicate with a slightly different name. I tried setting the field as read only but then the account can not be selected. 

I know I can merge accounts and contacts but I would like to prevent this from happening at all if possible.