Hi Alex,
When in the report there is an option to Customer this Report and open up the report settings. Near the bottom there is a header specifically called out for filters and an option to change that reports dynamic filters where you can set them to use the default dynamic filters (set in your tables report default settings), have no dynamic filters, or to use custom dynamic filters that you select. This allows you to pick out fields that are more relevant as dynamic filters and remove fields with so many choices they become less helpful. It doesn't allow you to alter the data in the fields to trim down options but can help you to assure a report has more useful dynamic filters. I hope this information is helpful Alex.