To leave the completely off-topic discussion of time-travel mechanics and get back to the subject at hand:
Well, we've already seen
- negotiate a group assault of humans and gods
- use robot nukes
- use coalition nukes
- negotiate a group assault of humans, gods, robots, and coalition nukes
- negotiate a slave revolt
- negotiate a group assault of humans, gods, slaves, robots, and coalition nukes
- outfox him on a negotiation
- use time travel, undoing the entire cataclysm
- somehow syphon off or using up the Earth's abundant magic, withdrawing his interest in the place and popping Atlantis itself back into limbo
which would all be storybook resolutions.
There are also the antiheroic plans for
- squirting tabasco sauce in his eye
- summoning elder horrors
- pulling Atlantis into the Minion War between Hades and Discount Hades
- bioengineering some superdisease
- turning his current untrusting partners (other splugs, sunaj, naruni) against him
The canonical answer in the Western tradition would be for a priest or pally to invoke the power of the Lord on High, whose continued but unstatted existence is hinted at in various ways. YGMMV.
The canonical answer in the Palladium tradition would be to smudge one of his transformation circles, turning him into Kalki, principled god of optometrists and fluffy kittens.
The canonical answer in the Multiversal tradition would be to park the SDF-1 or 3 over Splyn and turn him into a smoking crater with the reflex cannons (Damage: ALL).
The answer within the Atlantis book itself is to teleport his butt in front of an opened Azlum portal, converting him to scrupulous good alignment.
A
Rifter article details a variety of dimensional bottles and prisons, including a pet dimension of Splyn's.
Given that PCs use PB as their dump stat, finding one of the swimsuit editions of the
Rifter and using one of the NPCs there to seduce him.
Making contact with the Citadel of [Sirs Not-Appearing-in-this-RPG] and/or [Sir Not-Appearing-in-this-RPG] (C137).
Teleporting him into near orbit canonically has a 90% of instantly vaporizing him.
Setting up one of the CS antiteleporting systems, followed by a team of several dozen PCs rolling KO punches (everyone on Rifts is a boxer, retired boxer, or training to be a boxer), followed by Crimson Wall of Lictalon while he's stunned.
Death touch attacks.
Chi attacks.
A very long lecture from the Jade Emperor about what a naughty alien intelligence he has been, followed by chi attacks.
Vampires.
Various soul-drinking rune weapons.