The plot generated by "rlocus" has two sets of axes, one hidden and one visible. The labels that you see are linked to the hidden axes. Therefore, when you use "xlabel" or "ylabel", the changes aren't implemented completely and you can still see the "seconds^-1" in the labels. In order to rename the labels according to your liking, you can use the following set of commands.
axIm = findall(gcf,'String','Imaginary Axis (seconds^{-1})');
axRe = findall(gcf,'String','Real Axis (seconds^{-1})');
set(axIm,'String','Imaginary Axis');
set(axRe,'String','Real Axis');
The call to the "findall" command finds the exact handle of the axes labels that you want to change by searching for the "String" property that is set to the current axes labels. If you want to change the axes labels to some other string, you can then execute the "set" command.
