nope, sorry, once they leave then no reporting can be done. the person has to be "in the same district" as you in order to initiate a report.
also, on a side note, you do
not need to run up to the person in order to report them. simply click their name and type /report, or, a different means of doing so is:
press "/" (assuming you have "/" as a trigger to activate the chatbox with an auto "/" command), type in report *space*<insert player's name here>
or
press enter, type in /report *space*<insert player's name here>
~LeNa~