Gilles,
For more info about TQAs, you can search in SCN, where the topic has been discussed quite a bit. You can also refer to http://service.sap.com/sap/support/notes/488725 ; or the online help files at Displaying Temporary Quantity Assignments - Tools - SAP Library
In every GATP system I have ever worked on, there is a regular job running which deletes 'old' (unnecessary) temporary quantity assignments. However, it is important that you understand why they are not being deleted during standard processing before you begin deleting them.
Best Regards,
DB49