Check the job log to make sure it has a "completed" at the end. If so, there there has to be some reason why it wasn't removed (unable to connect to DB, etc). Maybe check the System Job Log to see (around the same time as the end of job) for additional errors that might determine why this happened.
To remove this from the Active Job Queue, you will need to stop/restart your GADirector Service.
Concerning the Thread-Safe...
If the project being run Is/can be run as a thread-safe job, then, from the Project Level in the Control Tab, change the "Thread safe" to YES. This will allow Multiple versions of the same project to run at the same time.
BUT ONLY DO THIS IF YOU KNOW FOR SURE THAT THE PROJECT DOESN'T AFFECT OTHER PROJECTS RUNNING AT THE SAME TIME!