You could try to add a filter in your lookup definition.
Assuming the field/column in your csv file that contains the users is called "user":
Open the lookup definition, check Advanced options, add
user!="urn:forms:anonymous#*"
under Filter lookup.
Hi
here is one example how you could do it https://community.splunk.com/t5/Splunk-Search/Where-to-find-unmatched-regex-events/m-p/440340. If you are not needing those on your lookup, then create it without those.
r. Ismo