Cancel a Spot Request vs Terminate Spot Instance
You can definitely cancel your spot instance requests via the Management Console and via the CLI. There appears to be a command to terminate the instance also.

See the following AWS docs as a reference:

AWS Docs - Cancel Spot Instance

As for terminating just the spot instance, I'm not sure but I think the, still active, spot request would spin up the instance once the spot price matches the request again.

** Edit **

Worth noting that if you terminate the spot instance within an instance hour then you pay for the full hour, even if you only used a few minutes of it.