The version number comes from the debian package version, so you can probably try the following:
- check with "apt-cache search mp-harmattan" what is the complete name of this package in your case (I guess something like mp-harmattan-341.2-pr)
- check the version with "apt-cache policy <name found above>"
If the 'Installed:' entry is different from 'Candidate:' entry, that may triggger the request to update. To avoid that you can trick the device that it has a 'newer' version like so:
- As root, edit the file /var/lib/dpkg/status (careful not to make mistakes, I suggest to make a backup copy of the file before beginning to edit)
- Search for a line that begins with "Package: mp-harmattan". Few lines below there is a line that begins with "Version:". Simply change that version number to something slightly bigger than what the update proposes, e.g. 40.2012.33-3
- Save the file and run "apt-get update" again