There's a lot of confusing techno talk out there, but this is how I did it on Windows 7.
Download a spy killer such as Spybot.
While that's downloading:
Go to: Control Panel/System/System Protection. Create a Restore Point and name it something like:"Before Blekko"
Go to : Contro Panel/Programs and Features. Look for anything in there with "Blekko" or "Spam Free". Also look for any Programs you don't recognize; if in doubt, do a Google search and check them out. Uninstall the suspect programs.
Go to your homepage and check Internet Options in Tools. Delete all info in the Home Page box. Make sure the bowser is on your preferred Homepage and select "Use Current".
Go to Start and open the search bar. Type "regedit" and open regedit.exe. Highlight the "Computer" icon, go to Export and save the file to safe location (maybe a disk or USB).
Go to: Edit and search for "Blekko" and any of the potentially unsafe programs you uninstalled.
As they pop up, delete them.
Now your spykiller is installed, restart you PC and constantly tap the F8 key until a menu appears: Select "Safe Mode" and enter.
Once booted up, run the spykiller. Claen up anything it finds.
Once that's done, run your normal antivirus program as well.
Reboot your PC, open IE and check that ALL refernces to Blekko have not reappeared. This includes a greyed out Blekko Toolbar sitting at the top of any Google search you may do.
Once satisfied all is well, Create another Restore Point and name it "After Blekko".
Do another export of your registry, making sure you delete the original backup.
Hope this helps!