At least in MKIV. I haven't tried MKII.

The following example should demonstrate this. With no mode specified on 
the command line, this should enable mode three and prevent and disable 
the other modes. It seems that \preventmode is not only ineffective in 
what it is described as doing, but also disables the following \disablemode!

Or perhaps I am misusing this or misunderstand what it should do. 
(2014-08-29 20:57 standalone)

    \definemode[one][keep]
    \definemode[two][keep]
    \definemode[three][keep]

    \define\ModeOne{nil}
    \define\ModeTwo{nil}
    \define\ModeThree{nil}

    \startmode[one]
       \define\Mode{one}
       \define\ModeOne{set}
       \disablemode[two,three]
    \stopmode

    \startmode[two]
       \define\Mode{two}
       \define\ModeTwo{set}
       \disablemode[one,three]
       \preventmode[one]
    \stopmode

    \startnotmode[one,two]
       \define\Mode{three}
       \define\ModeThree{set}
       \enablemode[three]
       \preventmode[one,two]
       \disablemode[one,two]
    \stopnotmode

    \starttext
    Mode is \Mode.

    ModeOne is \ModeOne.

    ModeTwo is \ModeTwo.

    ModeThree is \ModeThree.

    Mode \doifmode{one}{one}\doifmode{two}{two}\doifmode{three}{three}
    is active.
    \stoptext

-- 
Rik Kabel