Follow these steps to ban a user from your community:
1. Login as Administrator.
2. Go to user profile you want to ban.
3. Click cog icon on the right side and select ban user.
4. Pop up box will appear, select which option you want, to ban user forever or for a certain time.
5. Confirm the ban and the user will be banned now. User can not login into community and will have their name/username stricken.