Remove auto suggestion for usernames How do I remove this function?
It's up to the website to disable autocorrect on fields. You can disable autocorrect globally: go to System Preferences → Keyboard → Text and uncheck ‘Correct spelling automatically’.

Instead of disabling it, you can click the × to cancel that correction, and once you've done that a few times, the word will be added to the dictionary and won't be autocorrected.