Web site theat blocks loading content from certain sites or domains

I am worried that my web may be hacked (apache + joomla 3). Most of hacks consist on placing spam on posts or sending information to other websites in other domains/IPs. I am thinking about a security protection, maybe done with .htaccess that blocks any request done from my web site to some blacklist of domains. Or the other way around, allow only to fetch content from a whitelist of domains (localhost and very few).

Any idea how this can be done? Ideally it eould require only to change the root .htaccess file and be compatible with joomla SEF.