m0n0wall uses HTTP authentication. For every page you request from m0n0wall, your browser sends the username and password from its cache. There is no reliable way to force the browser to "forget" the username and password, and session management to work around that would introduce potential security vulnerabilities, so m0n0wall does not provide log out functionality. To safely log out, close your browser.
Your web browser may have a way to clear cached HTTP credentials. Check your browser's documentation for further information.