The arrows provide important information about the frequency response, and as a result there is not a way to disable the arrows. However, the following MATLAB Answers post provides a workaround: http://www.mathworks.com/matlabcentral/answers/242901-nyquist-plot-and-arrows
If you would like to change the number of or location of the arrows, you could first remove the arrows like above, and then add in arrows using the "quiver" function. The following MATLAB Answers post explains how: http://www.mathworks.com/matlabcentral/answers/260903-adding-an-arrow-to-a-polar-plot.
In the code at this post you can modify the "factor" variable to choose the frequency of arrows. You can modify the initial value of the "j" variable to determine the location of the first arrow.
