Can I close emt ell in wall?
Short answer:  no, you can't bury a pulling elbow, the cover must remain accessible.  

It may be tempting, but really it's better to find another way.  

As mentioned in the comments - you could add an access panel in the drywall, and make the pull elbow accessible. You could even hang a picture over the access panel.  You could install a pull box with the cover accessible.