I can right click on the column name to bring up the properties. 

Click on Remove and nothing happens (or click on apply after clicking remove - nothing).
Or bring up standardize attributes 

Click on remove and nothing again happens.

Spec limits have not been removed.... 
 
The only way it seems to work for me is scripting...
E.g., 
dt=current data table();
Ncols= ncols();
for (i=1, i<=Ncols, i++,
  colx = column(i);
  colx << delete property ("spec limits");
);