Avoid external Links

No it isn’t. CyberSyn does not provide content modification features. If you want to get a full control on syndicating content, you should obtain the professional version of CyberSyn: https://www.cyberseo.net/cyberseo-plugin/

Can you please tell me, where the option in professional version?

The feature is described in the manual: https://www.cyberseo.net/content-syndicator/#post_links_handling

However it’s just a bult-in function. In fact, the CyberSEO plugin allows you to take full control over the syndicating content by means of your own PHP code, which can be added right on the feed settings page, so a few lines of code will just anything with the original content (there are no limits at all): https://www.cyberseo.net/content-syndicator/#php_code