Explanation:

A. Use a validation rule

A validation rule is the simplest and most maintainable way to prevent users from editing a record based on certain criteria, like a specific stage value.

It’s declarative (no code needed).

You can write a formula like:

AND(ISCHANGED(StageName), ISPICKVAL(StageName, "Closed Won"))

Can also include NOT(ISNEW()) to block updates after creation if needed.

B. Use a trigger

An Apex trigger offers fine-grained control over record updates and is useful if:

Complex business logic or conditions are required.

You need to enforce this logic beyond UI changes (like API or integrations).

A simple before update trigger can throw an error if the Opportunity is in "Closed Won".