Locking Down Opportunity Customer Lookup to Accounts Only

By default, customer fields are open to accounts, contacts, and sometime leads. A common request is to lock down a customer field to a particular entity type or set of entity types.  For example, you may want to lock down the customer field on opportunity to only accounts (not a contact).  There is a simple fix to handle this.  Simply add this line to only allow accounts to be selected (entity type code of 1).
crmForm.all.customerid.setAttribute("lookuptypes", "1");

The problem is that users can get around this by converting an activity such as a phone call to an opportunity.  If you create your phone call from a contact then it will set the customer to the contact.  The fix is to add code to OnLoad that verifies the customer field.   If a contact is selected then it will reset the customer field to the contact’s parent account.  If they don’t have a parent account then I blank out the field.  Since this field is a required field on opportunity, the user is required to specify a valid account in the customer field before saving.