There is no way to prevent making additional changes to an image in the LR catalog. There are several things that you can do to achieve your goal.
You use color labels to indicate which images are complete and which images are complete and have been published. With these two colors, You can at a glance see which images you can touch and which you need to leave alone. Just because an image has a certain color label does not meant that you won't attempt to further refine the develop settings.
Hope it helps.
Thank you for the reply
What you mean by color labels and where I can find this option?
You can't lock the developed image, but you can take a snapshot of it, so that if you make any more changes (deliberate or accidental) you will still have the snapshot of the finished image.
Good answer Bob Frost.
There is a Plug-in (Donation ware) available that will automatically create a Snapshot whenever an image is exported.
See it at Jeffrey Friedl's site- Jeffrey Friedl's Blog » Jeffrey’s “Snapshot on Export” Lightroom Plugin