If a client no longer needs a service you are providing for them, you can easily untick it from the services tab on the client page.
Just click on "Edit" and untick the service you no longer need.
Once you have done this, no further jobs will kick off as part of this service.
If there are any remaining jobs showing on the Work tab for that client which belong to the service, you will need to manually cancel them.
Just tick to select the job(s), then click on Actions > Cancel jobs.