In D&D is there any situation where a player character could not be given a will save? Such as unconscious, insane, already under a previous illusion? If you are not in control of your mental capacities can you resist further attacks?
Yes, but you can choose not to exercise it. And sufficiently creative enemy mind controllers can convince you to not exercise your will save.
The relevant line from the srd is:
Which seems quite clear cut. Unfortunately, dominate person, states that:
Now, it also notes that "actions against [the dominated character's] nature allow another saving throw." Therefore, a sufficient bluff will be necessary to suggest to the target to voluntarily fail its will save. However, when combined with the diplomacy mechanics and the nature of Charm Person, there are theoretical situations where you can successfully order your "dearest bestest friend" to choose to fail their will save.
Whether or not this actually works is entirely up to your DM's reading of the spells in question and how he/she uses the social interaction rules.
A brief primer on mental takeovers, assuming plenty of time, resources, and a DM cool with the idea.
If I was an evil caster, here's how I'd do it (ignoring the mother cyst line of spells, which neatly circumvents most of the annoying bits).
As a wizard, beguiler, or bard, I'd start by casting glibness on myself. I'd then, using the suggestions from here, make sure I could use sleight of hand at an appropriate level and had the skill trick "conceal spellcasting." It is also important for this character to have the skill trick "False Theurgy." Since this character is an obsessive-compulsive mind controller, dedicated to his art, he would have a ring of silent spells. At this stage, I'm likely a mindbender/1, so I've got telepathy as well.
So, having cast glibness, I would start the process by casting charm person. I'd false theurge that into a prestidigitation or other completely harmless and believable spell. If the charm person fails, I would progress onto the next target. As enemies know if they've passed a saving throw and as I'm being extremely sneaky, I would want to do nothing to suggest a pattern of my presence and charm attempts. This attempt, of course, would happen at some sort of pub or party, If I had the feats free, I'd grab still and silent spell to do this sort of thing properly, though slight of hand reduces its need.
Once charmed, I would spend some time plausibly isolating them from their social support structure via skill rolls and roleplaying. Happily, the +30 to bluff from Glibness should help this quite significantly.
Once isolated, I would use bluff to establish a personae as a local diviner or astrologer, someone who could plausibly cast spells at the target as some sort of medical regimen.
I would mentally suggest (providing deniability that it came from me) that the target wants to treat me as an ally for cast spells, as I have some magics that could greatly benefit it. The mechanics of casting spells on friendly people is effectively the same: they choose to fail their saving throws and are therefore affected by the heals, buffs, or whatnot that the caster is putatively casting. With false theurgy, we can plausibly perform these "beneficial" spells.
Obviously, if any of these spells fail, it's critical to make sure the target doesn't believe that the spells were bad, that they came from me, and that they ever happened at all.
Once the first suggestion (to treat us as an ally and fail saves) is established, we then have to implant two critical long term suggestions via hypnotism. First: that we always should be treated as an ally with respect to casting spells, and second: that the character should always be willing to chat with us privately, as a friend.
The best part about this is that, because we're layering hypnotism on top of charm person, charm person changes their attitude to (at worst) friendly, and hypnotism causes an explicit two-step increase from that to fanatic. Thus, the character will always be willing to (even at the cost of laying down his/her life) have us cast spells at him/her, and to have a private chat with us. If we're being particularly unsubtle about the spells we're casting, we probably want to do a "believe all spells originating from us are helpful." But, if we're doing our jobs, that last one is unnecessary.
At this point, you can cast whatever you like at the target, and the target will always be willing to step aside and have you cast the spells at them. While, clearly, this methodology isn't something that will work in combat... it does accomplish the goal of depriving the target of his/her will saves.
Quick and dirty mugging
While "always" is a risky qualifier because if someone looks hard enough maybe they can find an exception, the normal rule is that you are allowed a will save against effects pretty much all the time. You don't have to consciously use a save, it's your innate ability to resist or shrug off a mental effect.
You get will saves while asleep or unconscious. You get them while insane. You get them while under a previous illusion (subject to the rules on will saves against illusions, of course). You even get them if your Wisdom has been reduced to 0, though in that case you will be taking a significant penalty to the roll.
In the case of Speak With Dead, you can even get a will save as a corpse.
This answer on a similar question about reflex saves has some rationale on why the game works this way, but it's true of all the saves.
If the spell allows a will save, there is very little that can happen to remove that defense.
And in the end, a saving throw, although automatic, still is a voluntary action. Players can fail them if they want to, the same way they can just not dodge a fireball if they don't want to. Find a way to force a character to drop his defense and you have a spell without a save. If the player character is tied to a chair, a dagger to his throat may have him lower his defenses real quick. If a charismatic, disguised enemy can persuade the character that this spell is beneficial, the character may forgo a save.