Hello Bob,
The Account types select box appears only if you have more than one account types. If you want to remove the account type select box from the Join page, you need to remove all accounts types. But as far as we've understood you need to have several account types, but do not want users to select the account type during the registration. 

The fact is that the account types is the core functionality and cannot be managed through the Paid Membership plugin. You need to make changes in the core functionality to hide this select box. You can search for the existing solution at the Custom Code Modifications forum: http://www.oxwall.org/forum/117 Or search for a developer who can assist you with this modification at Oxwall Market: http://www.oxwall.org/market/specialists.
If we took you wrong - please provide us with more details.