Play with an attribute named replicas to scale up/down your deployment.
To scale down
kubectl scale deployment my-app --replicas 0
To scale up
kubectl scale deployment my-app --replicas 1
To work with multiple deployment
Wrap it in basic for loop. This example will scaled down all deployment in the default namespace.
for i in `kubectl get deployment -o name`; do kubectl scale $i --replicas 0; done;