You have to disable the header and footer by setting the values to 0pt.

\setuplayout

  [backspace=15mm,

   width=middle,

   topspace=15mm,

   height=middle,

   header=0pt,

   footer=0pt]

\showframe

\starttext

\input knuth

\stoptext

You can also use the fullpage module when you equal margins for your document.

\usemodule[fullpage][style={margin,empty},margin=15mm]

\showframe

\starttext

\input knuth

\stoptext

Wolfgang

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