It's a shame that it's not easier to do this, but here is a Solution that explains how to do it:
Here's the code provided in that Solution.
% create the polar plot, and store the line's handle
p = polar((0:99)*pi/100, (0:99)/100);
% find all of the lines in the polar plot
h = findall(gcf,'type','line');
% remove the handle for the polar plot line from the array
h(h == p) = [];
% delete all other lines
delete(h);
% If you want, use the following lines to remove the text.
% (I prefer to leave it in place)
% find and remove all of the text objects in the polar plot
% delete(findall(gcf,'type','text'))
