Files detected as 'not-virus:BadJoke' are nasty jokes. These files are not destructive, however the things they do might be unsuitable for certain computer users.
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.
More information on scanning and removal options available in your F-Secure product can be found in the Help Center.
You may also refer to the Knowledge Base on the F-Secure Community site for more information.