Here is how it should work:
1 << slot should only disable removing Items, but allow replacing and placing
1 << slot+8 should only disable replacing items, but still allow removing Items and placing items
1 << slot+16 should only disable placing Items, but allow replacing and removing
To disable an armor stand completely, add them together:
10000000100000001 (binary)
Here is how it works at the moment: (1.9)
1 << slot completely disables all actions
1 << slot+8 disables removing and replacing, but allows placing Items
1 << slot+16 disables placing but allows replacing and removing
This is not good because it doesn't allow for some combinations, for example only allow removing but disable replacing and placing which could be used to detect right-click actions on armor stands.
It also doesn't support the off-hand at all, which should get fixed.