Refer to SAP Note 497090.
User status cannot be used to prevent automatic PR creation from the Project.
There are 2 workarounds:
1. When you attach the component you need to set the PR/Res indicator to NEVER. The material will still show in the planned cost but no PR or reservation will be created. When you are ready to release it change the setting to IMMEDIATLEY.
2. Use a PR release procedure which will prevent the PR from becoming a PO unril it is released. This is a MM function.