This URL deletes the calendar preference ceBgcolor from the user’s preferences.
http://calendarserver/set_userprefs.wcap ?id=b5q2o8ve2rk02nv9t6 &del_attrs=ceBgcolor
If the attribute to be deleted is multi-valued and there are other instances of the preference, only the first instance encountered is deleted. To remove all of the instances of this preference, multiple set_userprefs commands must be issued, one for each instance.
For example: After running get_userprefs, you see there are two values listed for icsSubscribed. To clear both of them, two commands must be issued: