(This is a bump, with an improved example, of my earlier note on the 
subject.)

In MKIV, \preventmode does not work. It appears to do the opposite of 
what is intended, and enable the specified mode. It also does not 
prevent the explicit enablement of a mode (as can be seen by 
uncommenting the commented line in the MWE).

I have not tested it with MKII , where it might work. (Will ConTeXt 
online ever be restored?)

This goes back to TL14 and likely earlier.

Do I misunderstand what \preventmode should do, or have I misused it?

And why are there {curlies} instead of [squares] around the mode name in 
\doifmode and friends?

MWE, run context MWE.tex:

    \starttext
    \preventmode[X]
    %\enablemode[X]
    \doifmode{X}{!}
    \stoptext

-- 
Rik Kabel