Install FormulaLock

About one minute, once per spreadsheet.

1. Open your Google Sheet. Go to Extensions > Apps Script.
2. Delete anything in the editor, then paste the full code from formulalock.com/FormulaLock.gs (open it, Ctrl+A, Ctrl+C).
3. Rename the project from "Untitled project" to FormulaLock (click the name at the top), then click the save icon. Close the Apps Script tab and reload your sheet.
4. A FormulaLock menu appears. Pick Lock selected cells. Google asks you to authorise the script the first time: click OK / Review permissions, choose your account, then on "Google hasn't verified this app" click Advanced > Go to FormulaLock (unsafe) > Allow. Google shows "unverified" for every script you paste yourself that hasn't been through their app review; you can read every line of the code first.

Pro key

After buying, use FormulaLock > Enter Pro licence key in each spreadsheet where you want unlimited tabs.

Uninstall

Unlock your cells (FormulaLock > Unlock selected cells), then delete the code in Extensions > Apps Script.