Custom form template without any region
All you need to do is search your page.tpl.php for parts like:

print render($page['region_name']);


and then wrap them in appropriate if construct, not rendering them at all on pages you don't want to see them rendered.

Alternatively, you can create files named page--user.tpl.php, page--user--login.tpl.php and so on. See this answer for details. Simply copy default page.tpl.php and remove unneeded regions from a copy.