You can use entityManager.clear() to detach all (!) entities from the entity manager. Changes to detached entities will only be written after a merge to the db.
A much cleaner soluction (IMHO) to the problem are Seam's long running transactions.
thanx for the replay.
the deadline of the project is too tite that I don think I hav time to learn Seem's new feature.
I'll try the entityManager.clear() method.