Can Immovable Object target held or worn object on your opponent?

Can Immovable Object target a piece of armor or the weapon of a target creature?
The spell says:

You touch an object that weighs no more than 10 pounds and cause it to become magically fixed in place.

For example can I touch a guards helmet and make it immovable, in order to restrain him? If so is there a save or is it a simple spell attack?