Open Taskmanager, find wcgridHCMD.exe and terminate process. You will get a new job, so change your profile if your dont want HCMD before cancelling or simply exit the Agent (right click on icon in right hand bottom system tray on start bar). The job is valid for 2 months from download date.

There are no Beta's presently running. They are announced in the Beta Forum Area and for the moment is only an option working for BOINC. If selected, and a machine has proven reliable, will Beta test work be send to the client, automatically. Usually the return time is very short, not like the regular BOINC jobs which have a current standard due time of 9 days.

Added: You could keep your WCG(UD) agent running and subscribe in the My Forum Profile, My Forum Watch for e-mail notifications from the Beta area. At that time just exit the WCG agent and start the BOINC agent. BUT, as said, only reliable BOINC clients get beta work.... Now, usually beta work often goes in batches of 500 with 4 copies, i.e. 2000 total, so often it's over before you notice on Windows or Linux or Mac, each run separately or concurrent.