At the bottom of each game you have a link to start a report, that way you report directly to the staff.
Additionally, you can contact Itch supportand report the user directly.
edit:
When Itch deletes that user or their content, it will eventually stop appearing on Google.
There's not much else you can do beforehand, unfortunately it's impossible to protect the code from theft. Only subsequent measures such as reporting to DMCA to have the page deleted can be taken.