Dynamics CRM: Disabling the selection of contacts for opportunities all the way

I recently got a requirement for a Dynamics CRM implementation that all recorded sales should follow the B2B (business to business) and therefore, opportunities should only able to be related to accounts, and not contacts as well.

This is a common scenario which can be easily covered with some simple JScript in the opportunity form. However I found out that in some circumstances, an opportunity could still be created for a contact, thus violating the requirement and even worse, breaking some of the implemented processes and business logic. Here is how I fixed this issue.