Placeholder Users
In our main app we have around 4,000 placeholder users in addition to our regular and denied users. ~8 of them have actual names of real users. Most of them are simply a random number like this: ""-5...
Forum Discussion
I know this is old, but here is what we have learned so far about the User type fields behaviors in Quickbase.
If a User or User-List type field is written to via csv import or upsert and the Username in the data values is not found (for whatever reason) a placeholder is added. The best way to add a User to a User type field is to write the email address and not the Username. Usernames or Full Name are fickle and require exact matches. Take for instance my name Jim Harrison. I upsert to a User Type field Jim Harrison and get a placeholder. Upon further inspection I find in Quickbase my name is Harrison, Jim. In order for the match to locate the correct name, the values must match exatcly. The email address is much preferred for this reason.
Alternately you can uncheck the "Allow users to create new choices". Unchecking this setting can cause other problems in the scenario described above. You may not get placeholders but those records may not be created or the import can fail with an error.