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.
Depending on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the suspect file, or ask you for a desired action.
More information on the 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.