Damp down
Definition 1 of 3
Pro Tip 1/3Non-physical action
Used metaphorically to describe reducing the intensity of actions or emotions, not just physical things. 
The government acted swiftly to damp down the rumors.
Pro Tip 2/3Not silence
Doesn't mean to stop something completely, but to make it less intense. 
The new policy helped to damp down the public outcry without silencing the concerns.
Pro Tip 3/3Often negative
Commonly used in contexts where something unwanted is being made less strong. 
The principal's announcement helped to damp down the tension among students.