Computer too accessible.

You can exclude the particular user or Mac in the System/General/File Sharing settings pane:

Just don't add the user you want to exclude.

NOTE: This screenshot is from the Ventura system. If you're running an earlier system it will be in the System/General/File Sharing preferences pane and will look different.

You can always call AppleCare for assistance. You will need the SN of the computer which can be found by clicking the  symbol in the upper left of the display and then clicking About this Mac . You can find the phone number for your portion of the planet by clicking AppleCare Contact Info and they should be able to assist.

Your post does not make sense the way it is currently written. If you mean the iMac has 3 accounts on it, then you need to either eliminate the account you are having problems with or make the login password extremely difficult and secure that password so you do not lose it!

You can adjust the write access that each of the others Mac’s have to your iMac.

see > Set up file sharing on Mac - Apple Support