Draw a white disk above the disliked area as follows
In[1]:= s = Import[FileNameJoin[{NotebookDirectory[], "..", "Abt_skrot", "tension.xlsx"}]];
In[8]:= Show[{ListContourPlot[s, Contours -> 10, PlotLegends -> Automatic, DisplayFunction -> Identity],
Graphics[{White, Disk[{0, 0}, 2, {0, \[Pi]/2}]}, DisplayFunction -> Identity]},
DisplayFunction -> $DisplayFunction, AspectRatio -> 1]
to see

Ask yourself whether this graphics is cheating because the shown inner boundary does not fulfill any boundary condition, right?