Current problems:

• Cannot enable the offhand slot
• Cannot disable the mainhand slot.
• Can replace when placing is disabled.

The current implementation is:

1 << slot = disables all interaction
1 << (slot + 8) = disables removing/replacing
1 << (slot + 16) = disables placing, allows removing/replacing

Slot values:
boots slot: 1, pants slot: 2, chest slot: 3, helmet slot: 4
offhand is always disabled, mainhand is always enabled.

To reproduce:

• /summon armor_stand
• /entitydata @e[type=armor_stand] {DisabledSlots:255}, for instance (more example values below)

Example values:
Disabled all interaction, except boots: 11111101= 253
Disabled all interaction, except pants: 11111011= 251
Disabled all interaction, except chest: 11110111= 247
Disabled all interaction, except helmet: 11101111= 239
Disabled all interaction, all slots: 11111111 = 255
Disabled removing/replacing, all slots: 11111111 00000000 = 65280
Disabled placing, allow removing, all slots: 11111111 00000000 00000000 = 16711680

What the implementation should be (see Searge's comment here):

1 << slot = disables all interaction
1 << (slot + 8) = disables removing
1 << (slot + 16) = disables placing

'Replacing' should theoretically not be possible, and the first flag should also disable placing and not just removing.