Go to System--->Administration--->Synaptic Package Manager
Enter your password and clik ok
Once it opens you should to similar to the following screen
Now you need to enter your package name in serach box in this example i am searching for midori
Now go to Package select Lock Version
Once it locked you should see lock icon next to your package
Now you need to click on reload after that should see similar to the following screen
That's it now you have successfullu locked midori package from upgrades.
Note:- Whenever you upgrade to next version of ubuntu you need to release those locked packages for this just use the same procedure as above and uncheck the packages.