What is OSD?
OSD stands for On Screen Display and it is the menu that controls the monitor settings. It sounds like somehow your menu button is stuck in a loop, which is causing the locked/unlocked messages to continue to flash. There are a few things you can do to solve the problem, some more drastic than others.
Reset the OSD
The easiest solution to try is to reset the on screen display in the menu. Hold down the Menu button (it is probably marked Ok) for at least a minute. A menu list will appear on your monitor screen. Using the adjust button, press the arrows to enter the menu. Scroll down to the Reset section. On the right side of your screen, you will see an alternating Yes/No choice. When the Yes message flashes, press the Ok button. The menu should be reset and the flashing OSD control message should disappear.
Fix the Menu Button
If resetting the OSD doesn't work, then you probably have a physical problem with your Menu button. Sometimes, particularly on older machines, the connections to the circuit board are dirty or failing. Because you have to actually get into the monitor itself, you will probably void any remaining warranty by attempting this fix. If your monitor is still under warranty, contact the manufacturer to see if there is a local repair shop that can help you.
If you want to tackle this issue yourself, grab a screwdriver and take the plunge. Remove the casing on the monitor itself and locate the contact switch for the menu button on the circuit board. Sometimes, just working it with your fingers or a screwdriver is enough to unstick it - you can try to clean off any dust or grime that has accumulated around it to see if that is the cause.
Disconnect the Menu Button
If this doesn't work, disconnect the button all together. This will definitely solve the problem, but you will no longer have access to your menu options. Given how many times over the lifetime of a monitor you actually access your OSD settings, this may not be such a hardship. You need to weigh the inconvenience of not being able to access the menu with not having that annoying OSD message flashing in your face.