Trying to prevent an opportunity from moving beyond a sales stage if account is not on approve clien

Hello.

I am looking to see if there is a way to make sure that an opportunity does not progress beyond my 1st sales phase if it is not an account on our target list.  So I am kind of looking for something that will look on the opporunity page and not let it save if the account type field is either null or "Not on Target List."

My initial thoughts are a workflow rule, but I am not sure how to proceed.

Any help is appreciated.
Thanks.
Glen