Incidentally, the minimum DC on that Symbol of Insanity is (10+lvl 8 + minimum Int bonus 4) 22, and more likely to be 26; that's assuming Xykon has no way to boost the DC higher (feats, spells active, etc). A 10th level Paladin with a 18 Cha has a (+3 base save, +2? Wis bonus, +4 Cha bonus) +9 Will save - they'd need to roll a 17 to save, and more likely higher (some wiggle room there for magic items that boost Paladin saves, etc). And I'm not guessing that more than a few are 10th level.
Hallow could give the Paladins a +2 save versus the Symbol (Magic Circle vs Evil). Consecrate wouldn't really do anything (-2 to Xykon's saves! Woohoo! +6 to turn him! Good Luck!). As others have pointed out, Dispel Magic is of no use in this scenario.
To trigger the Symbol, the paladins just have to look at it - not even read it, which was my first question. I'm sure that Xykon used a Mass Suggestion in there to get them to look. All it takes is one to trigger it. The ball bounces, so if it bounces far enough and then back, the Paladins might have to make multiple will saves, leaving them well and truly screwed.
The paladins had decades to prepare for attacks, but not decades to prepare for this particular attack. If it were up to you, how would you protect the throne room from a high-level sorceror or wizard?