The incidents generated in NNMi console will not get cleared/ reduced automatically until it's incidents has been reduced from the database by using nnmtrimincidents.ovpl.
You should use the command: “nnmtrimincidents.ovpl -trimOldest 20000” to trim the oldest incidents. You can repeat the command as many times as needed. Once its incident get removed from database, it will clear in Incident Browsing immediately.
You can find the syntax of the nnmtrimincidents.ovpl in the following docs