What are all the options for mental protection to stop another force user from manipulation?

Well there's quite a few, though the most obvious is to be a PC or character that's important to the plot. That requires the force power be rolled opposed to you instead of merely fueled.

Discipline is a central skill for force-users, and it's also used to resist most of those powers, so it pretty much happens by default.

There is a Force power that lets you shut down other Force users trying to use Force powers during an encounter. I forget the name of it, but if the "manipulation" you are referring to is Force related, like Influence, that power would directly oppose it, potentially shutting it down entirely.

If you're talking about manipulation via Misdirect, illusions can be dispelled by the Seek basic power. Otherwise, Discipline is the primary skill I would use.

3 hours ago, KungFuFerret said:

There is a Force power that lets you shut down other Force users trying to use Force powers during an encounter. I forget the name of it, but if the "manipulation" you are referring to is Force related, like Influence, that power would directly oppose it, potentially shutting it down entirely.

Suppress it is in the Guardian book.

Yeah that's the one, Suppress.

There is also the Slippery Minded talent, in the Shadow tree.

Willpower, discipline, slippery minded talent, and suppress power are the only ways I know of to protect from Force powers used against you.

I will say that the rule on Force Powers against important NPCs or Player Characters says by default you make an opposed Discipline vs Discipline check but the rules allow the GM to determine which skills are used in any case, so there may be times where you're resisting the Harm power with your Resilience or resisting the Bind power with your Coordination to dodge at the last second.

Personally, I have mainly kept it as Discipline vs Discipline when I ran games. That way I don't take away from the players that ask how to defend against the Force.