Is there an option to to remove the HitHighlightedSummary and default body managed property from free-text index?
If that's not possible is there an option to clear the values of this properties?
You didn't mention if you are on-prem or online. Either way, I would not remove the Managed Property. Much of the functionality in the search center is driven by that field. You cannot explicitly clear a single MP, well, not easily. But you can reset the entire index. (In the Search Service Application on-prem.) If you are in SharePoint Online it is more difficult. I am assuming someone saw "something" in a search result they did not like. If that is the case, just secure the content and it will only appear for the right folks. Search is always security trimmed.