There are 2 main ways to remove a contact from a job.
First is to navigate to that contact's record and select the status drop down on the job you'd like to remove them from. The very last option will be 'remove.'
Next, from within the job dashboard, you may drag and drop them to the 'remove' option. This will most likely display towards the bottom of your screen when starting to drag that candidate down. This may be done in bulk.
To add, you may use the plus sign next to any workflow stage to have the ability to import recent contacts, find a specific candidate, or import resumes.
You may also do this from the contacts record by select the 'add to' option on top of the page.