You cannot purge the notifications directly. I think you mean that you want to close them, right?
For FYI notifications, you can go to the full notification list, check them one by one and choose close.
For those that require a response, you must open each one and choose an action.
If you want to close a bunch of them without opening each on of them then you can use and probably modify the solution in this blog post:
To prevent the "Local event ERROR" and "Error in Workflow IGSPE002" notifications then you should follow the below MOS documents:
Error Notifications Received From IGS_PE_WF_GEN Workflows (Doc ID 1902783.1)
How To Disable AP Business Event "Oracle.Apps.Ap.Payment"? (Doc ID 1539027.1)