Yes, it is very easy to delete an individual and his or her descendants.
First do a new backup, because the process is irreversible.
Then click File > Export > Partial > Select Select them as if you were going to Export them as a gedcom file, BUT when you have selected the right people click DELETE instead of OK. And they are gone!
The same process can be used to prune unwanted ancestral branches of a family tree. (For example a newly-discovered will proves beyond a doubt that your ancestor Maud Roberts is the daughter of John and Emma Roberts, and not of the well-researched but unrelated Henry Roberts whose lineage goes back to Charlemagne.) Just remember that it can only be undone by restoring a backup!