D&D 5E Some 5E Rule questions

[Celtavian]

You can be immune to a spell slot used for divine smite?

Not that I'm aware of. I have seen no rule to indicate that a spell that protects from spells of particular level would protect from Divine Smite.

 

[Quartz]

You can be immune to a spell slot used for divine smite?

How is it different from any other touch-delivered spell?

 

[77IM]

It's not a spell. It's not on any spell list, doesn't have any casting time, duration, etc., it's not an evocation or anything. It's just a paladin special ability that happens to consume spell slots.