All the users pages are at /user/Username so you should put this in your robots.txt:
Or if your Q2A installation is in a subfolder like example.com/qa/ put this:
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.