Filtering

If you can work with PHP I recommend the filter hook 'rpwwt_widget_posts_args'. It gives the asigned function an array of the query parameters. Your function adds the parameter to exclude the author from the query and returns the extended array.
Helpful resource: parameters to exclude authors from a WP query.

If this is too technical for you try my versatile post list creator Ultimate Post List for free. The feature for exckuding authors via a simple dropdown list is available in Ultimate Post List Pro for purchase with a 14 days money-back guarantee.

Thanks Martin for the tips. I can do it, no problem.

Great. I will be glad if you’d tell how it works or write a review.