App Account in User Picker Fields

[Hugo Oliveira]

Recently an unknown user started appearing at the top of user picker fields, like Reporter and Collaborator fields:

Using the REST API ".../rest/api/2/user" I could retrieve only the accountID, displayName, accountType ("app") and groups, which didn't help much.

Is there a way to identify to which application is certain app account related?

Is there a way to prevent app accounts to appear in user picker fields?

Thanks