Delete share function in Google Sheet

They can make their own copy via the file menu:

or (ask you to) change the share settings with the share button in the top bar:

Just as you can add viewers/editors there, you can remove them as well, as shown in §3.2 here:

Stop sharing a file or folder you own:

1. Open the homescreen for Google Drive, Google Docs, Google Sheets, or Google Slides.
2. Select a file or folder.
3. Click Share or Share Share.
4. Find the person you want to stop sharing with.
5. To the right of their name, click the Down arrow Down and then Remove.
6. To save changes, click Save.