When using InfoPath forms as Content Types you're bound to run into the error "Content Type is still in use", when trying to remove an existing InfoPath as a Content Type.

From my experience you will not be able to remove any InfoPath form Content Type from a Form Library that has submitted InfoPath forms based off that Content Type.  If the library is using multiple InfoPath form Content Types you can go into the Form Library Settings page and update a property for it not to show.  From the settings page locate the "Content Types" section and then click the "Change new button order and default content type" link.

From the "Change New Button Order and Default Content Type" page uncheck the "Visible" check box for he form you no longer want appearing on the Form Library "New" button.

If this InfoPath form is the only Content Type for the Form Library then you can go and edit the Form Library security and not allow any new forms to be added.  The Form Library security link "Permissions for this form library" is located on the Form Library Settings page under the "Permissions and Management" section.

Earlier this year, I also discovered that you can remove the "New" button from the Form Library toolbar by blanking out the library form "Template URL". This property is located on the "Advance settings" page and is only available when "Allow management of content types?" property is set to "No".  I'm not sure why you're allowed to do this but I thought I would share it because it was something interesting.

Cheers