Get an Actual Magical Contract
The contract of Nepthas (Complete Arcane 148) (1,400 gp; 0 lbs.) leaves
[t]he details of the contract... blank, and the user can fill it in with any instructions, agreement, or conditions he cares to create. When the contract is signed, though, the item’s true power is revealed to both parties, and any signatory who breaks the contract is subject to a curse that strikes it blinded, deafened, and mute (no saving throw, though spell resistance applies). The curse of a contract of Nepthas can be removed only by means of remove curse cast by an 8th-level spellcaster or a
break enchantment spell (DC 25).
Because a contract typically involves two parties agreeing on a set of conditions, nonspecific terms might allow a clever signatory to escape them without suffering the contract's curse....
Contracts [n.b.--with a grain of salt--in general as the word's not in italics; who knew such vital information was in a splatbook's magic item description?] signed by creatures under the influence of charm or compulsion effects are null and void.
That more than one creature can be a contract's signatory is implied by the last paragraph, but this isn't stated, so ask the DM. Further, if the DM says that's possible, the DM must also determine what happens after all the bandits sign then one bandit breaks the contract. (However, the number of signatories doesn't appear to change the contract's power--the contract's effect is never discharged--, rendering a lone, rebellious, contract-breaking bandit affected by the curse yet leaving unaffected the other bandits unless they later individually break the same contract. Again, ask the DM.)
Having a contract of Nepthas means planning ahead, but the item's priced reasonably and doesn't require a caster to complete. A wealthy, lawful kingdom might equip trustworthy elite soldiers with contracts of Nepthas, urging them to impress defeated enemies into the kingdom's ranks.
The only real disappointment is the ease of removing the contract's curse. Although the contract is created at caster level 11, the contract's description appears to be written so an adept (DMG 107-8) capable of casting the appropriate spells at all can use the spells remove curse [abjur] (PH 270-1) or break enchantment [abjur] (PH 207) to relieve the consequences of breaking the contract. A sufficiently high-level adept can sometimes be found in a large town but more often a small city (DMG 139). The curse has no saving throw, though, and being blind, deaf, and mute is pretty severe, so first getting the adept to agree then breaking the contract is a good idea.
Because of the caster level at which the contract's created, a reasonable house rule to remove the curse sets the minimum caster level to 11 for the spell remove curse and the caster level check DC to 22 for the spell break enchantment. This permits scaling up the contract if a more powerful contract is desired.
In trying to determine the identity of Nepthas, I learned that in Advanced Dungeons and Dragons, 2nd Edition the contracts of Nepthas (originally from Tome of Magic (1991) but reprinted on page 329 of Encyclopedia Magica, Volume 1 (1999)) required signatures from both parties, allowed any reasonable number of signatories, required a deadline to be set, released from the contract creatures that die, prohibited forged signatures, and used the same example ("Slay a dragon in the Northern Hills") as Complete Arcane uses over a decade later.
And I still don't know who (or what) Nepthas was.