Currently the "Created by portal user:" filter is basically impossible to use if you have more than a few portal users. It's not in any order and you can't search within it.
Also would be helpful to have a "NOT" option just like all the other filters should have so you can do a negative search, like only show the ones not created by this user or these users (i.e. wanting to filter out the ones created by users of a certain domain).
This idea's status was out of date and has now been updated to 'shipped'. The report filters for 'Created by' columns are now searchable within the filter dropdown. It is also possible to configure the filters as 'not' by using the advanced filter option.