**Yes, I would argue you are immune.**

While this is not explicitly called out by the description of the staff, I think the text of the staff's effect and the spell lean heavily toward your strategy working (presuming, of course, you picked the proper subtype for your casting of *Protection from Evil and Good*).

As you have pointed out, the spirit in the staff is explicitly attempting to possess the user, and *Protect from Evil and Good* explicitly protects against possession. This seems quite reasonable even though the staff did not call out this spell specifically in its description.

As additional support, the staff's description says that *Dispel Evil and Good* can be used on the affected creature to exorcise the spirit and force it back into the staff. The effects of *Dispel Evil and Good* and *Protection from Evil and Good* have significant similarities, with Dispel being a higher level with some additional affects, and both share the line about ending possession. Therefore I would expect that if Dispel is meant to be able to use its effect to dispel the spirit, Protection should be able to use the similarly-worded effect to protect from it.

These two points together make me think that *Protection from Evil and Good* should be sufficient protection from the spirit of the staff attempting to possess you.