Only active forms appear on all your iPads. Once you archive a form, it will disappear from all your iPads. So, to remove a form from your iPad(s), you need to archive it.
How to archive a form
Visit the "My forms" link, after signing into the web portal.
Select the form you'd like to archive.
Click the "Archive form" button in the right-hand sidebar.
Whoops — I didn't mean to archive! How do I get it back?
It's almost the same process, just unarchive the form.