This interesting issue came up for one of our customers when we upgraded their environment from CRM 2011 to CRM 2013. After upgrade they decided to go ahead with 2013 layout of the forms. Hence we de-activated the Information forms that came from 2011 and made sure that end-users use the CRM 2013 forms. For e.g in the below screenshot, for the Account entity, the Information form is disabled and only the form named account is visible to the users.
However our customer was using one third party managed solution from 2011 and they have released a new version of the solution and that solution was still using the information main as active form. Hence when we imported the solution into our environment, the information form got activated again and that form started showing up to the users.
So the trick is to remove all the security roles associated to the form apart from de-activating it.
So even if the form gets activated unknowingly as in our case, it would still not show up to the users.
Hope this helps!