Subscriptions are cancelled with a regular RPC call with eth_unsubscribe as the method and the subscription ID as the first parameter.

Subscription ID - (required) The subscription ID, as previously returned from an eth_subscribe call.

{
    "jsonrpc":"2.0",
    "id": 1,
    "method": "eth_unsubscribe",
    "params": ["0x9cef478923ff08bf67fde6c64013158d"]
}

Result - true if a subscription was successfully cancelled, or false if no subscription existed with the given ID.

{
	"jsonrpc": "2.0",
	"id": 1,
	"result": true
}