This will not work in the following cases.

For Instance:

1. If we set our site with www format and create too many spam links.
2. Later on, we convert our site without a www format redirect.
3. Now, if we set RewriteCond referer for specific URLs or Domains via .htaccess, in that case, the old format www base site's backlinks will redirect and not show 403 forbidden; all reference links will work and reach the site.

Details Example:

www.abc.com (10 Spam Links or Domains) abc.com (10 Spam Links Domains)

Note: Suppose our Current domain is abc.com, and www.abc.com has been redirecting to abc.com.

In the Above Case, only the abc.com referrer will be blocked and show a 403 error, whereas if you also block www references URLs or Domains, these links will not be blocked for referral traffic.

If someone has a solution to this, please share.