The way to do this is to setup a second job which runs at 7am, and looks to see if the job is still running. If the job is still running then use the sp_stop_job to stop the backup job.
It’s best to let backup jobs complete as if you don’t then you don’t have a good backup in case something goes wrong.
Does your backup often take until 7am? If it does you may need to look at where you are backing up to as it may not be fast enough.