If you are running OS X 10.6.x or Lion, running Software Update will remove Flashback if it is present.
If you are running OS X 10.5.x, download and install the utility on the following page:
How to use the tools:
1) Download FlashbackRemoval.zip to the Mac machine you want to scan.
2) Double-click the zip package to unzip it in the current folder.
3) Double-click the FlashBack Removal app to run the tool.
4) Follow the instructions to check your system and clean any infections.
Where can i get more information about?
About Flashback malware: http://support.apple.com/kb/HT5244
About Flashback malware removal tool: http://support.apple.com/kb/HT5246