For churn meaning "to stir vigorously":
* Still
* Quiet
* Calm
* Settle
* Unmixed
For churn meaning "to turn over and over":
* Remain
* Stay
* Stand still
For churn meaning "to cause agitation or unrest":
* Soothe
* Calm
* Pacify
* Harmonize
* Stabilize
For churn meaning "to produce or generate a large amount of something":
* Decrease
* Reduce
* Diminish
* Stop
* End
For churn meaning "a process of rapid change or upheaval":
* Stability
* Stability
* Stagnation
* Stasis
* Continuity
It's important to consider the specific context in which you are using the word "churn" to determine the most appropriate antonym.