Methods for blocking over-the-air (OTA) updates are discussed in the Kindle Developer's Corner sub-forum. One method is described in step 7 of the first post in
this thread.
The recommended method has changed over time because Amazon tries to prevent this type of update blocking. There has not yet been an update released for 5.8.9.2 and so it is not yet known whether the current update blocking method will turn out to be effective for that firmware.