It is mentioned in Warhammer 40K that a Daemon can be killed if after being summoned it’s connection to the Warp has been severed. Could the same thing be done to a Chaos God? I say yes though it would be complicated.
The Chaos Gods are the realms themselves so to kill them you would have to summon vital chunks of it’s realm into the Materium then sever their connection to the Immaterium then kill or permanently render non-psychic the Avatar of the Chaos God.
End result: Anything left of the Chaos God’s realm would be too disconnected from the rest to retain the Chaos God’s sense of self and either become multiple lesser Chaos Gods or be absorbed by the other Chaos Gods.
The easiest Chaos Gods to kill with this method are Slaanesh who has less territory than the others and Nurgle who is also extremely lazy to the point where he probably wouldn’t do much to defend himself.
Incidentally in Warhammer Fantasy(which supposedly has the same Warp as Warhammer40K) Slaanesh has already been captured which led to his realm being absorbed by the Horned Rat(Chaos God of Vermin, Cheese, Decay and Death to counteract Nurgle’s role as Chaos God of Disease, Love, Plants and Life)… The Horned Rat seems suspiciously similar to the Tyranid Hive Mind…
Take out those 2 Chaos Gods and we see a pure war between brute force and cunning with change.
Khorne would have to be next and require multiple attempts at backstabbing the advisers that would have been sent by Tzeentch(the plans as agreed on by the advisor would have to have killing Khorne be in the early part of the plans).
If Tzeentch were the only Chaos God left he would work with the faction killing Chaos Gods in order to set up multiple attempts to take down himself.