in my case a project has a technical team, so project content type has a user reference field which is created by Entity reference module. but in node creation form, an "Anonymous" user is available to select.
how can I remove it?
One way would be to create a View with an "entity reference" display to show all users with an id greater than zero, then use the Views "Entity selection mode" for your entity reference field:
You can choose the View you just created and the anonymous user will no longer show.
There's also the Entity Reference View Widget module which provides an advanced version of this functionality, though it's still in alpha.