This Uninstall Plug-In… button will prompt you to remove any SpamSieve Mail rules, then uninstall the SpamSieve plug-in and launch agent. The SpamSieve application and training data files will remain. SpamSieve will then quit Mail itself so that the plug-in code is unloaded. It is normal for this button to be enabled even if the plug-in file is already uninstalled.
For more information, see the Uninstalling SpamSieve section.