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