I'd like to run bonic only casually, but when I close the bonic manager the bonic process still running in background. This didn't happen in old version. It's a bad idea to leave the processes there.

Then enable the exit dialog if it is disabled.

BOINC Manager->Advanced view->Advanced->Options.
Check "Enable Manager exit dialog?"

Then when you go File->Exit, you get

- Stop running science applications when exiting Manager
- Remember this decision and do not show this dialog

Check the first one only, so each time you exit BOINC Manager you get the dialog.
Or check both and when you exit BOINC Manager you will also quit running Boinc.exe and any science applications from projects.

Do know that by default the exit dialog is enabled.

Boinctray.exe will continue to run. This is the idle checker program for BOINC under Windows Vista and Windows 7. It starts from the registry, not when you start BOINC.

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Oh, that worked! thanks a lot.
My suggestion, put "Stop running science applications when exiting Manager" to the Options dialog as well is a good idea!