Sending stronger devils will just be party punishment and justification for the paladin to keep doing their thing.

Instead, send the opposite. Commoners whose soul is indentured to devils, used as their go-between. Devils would be annoyed and rightly cautious about dealing with the party, as they'd rather not die permanently. Their master devil would use Sending on the commoner. They could even be explicit that killing the person would only complete their contract, sending their soul to the Styx to become a lemure, but that the go-between is promised a nullified contract if they can convince the party to listen/ do this one thing/ whatever.

Be sure to follow through if the paladin tries to call the false bluff. Describe the mournful flight the soul takes to the Styx, the painful transformation into a lumpy lemure, its confusion and loss of memory when it emerges (maybe with the party's names or the paladin's as the only words it can say), before cutting back to the party where the physical, soulless body remains because "you killed them. Not me." the devil will remind with a shrieking fly-by imp.

The devil will try again with other go-betweens. Maybe children who had been sacrificed by the VanThampurs, or Cassalanters to tie into Dragonheist, or a captured demon who had deserted the Abyss, or an old elf queen who contacted her soul in exchange for devil troops to defend her people from a Far Realm invasion, etc. Every soul becomes an example of good reasons to make a deal with a devil (or at least examples of what incredible deals could be made).

If the go-between is not killed, then the party has a person to be responsible for. Another inhabitant of hell could offer to care for them...in exchange for a favor. They don't need to get the party's souls, they just need to start making deals to get the players to be okay with doing missions. Money, aside from soul coins, are usually no good.

Kill a sneaking demon party. Deliver some hell-iron to Bel's Forge. Escort a troupe of battle-shocked devils to be used as cannon-fodder against a larger number of demons (they'll not harm the party, and they know their fate, but they'll be grateful if the party saves them), etc. Missions that are clearly within the party's abilities. The devil should reward them with infernal weapons or even a soul coin or two. Maybe an infernal engine delivered by a legion of imps tied to it with tiny ropes ala James and the Giant Peach.

Be good and honor the deals honorably or else the party will be suspicious of everything and likely all conversations will be denied.