Take a look at the accounts. In my case they all start with "_spo". Open your search center and edit the People Search results web part. Add an exclusion to the query like
-accountname:_spo*. in my environment that excludes them. Another alternative is to copy the existing People Result Source and edit your copy adding the above exclusion. Then change the Result Source that the web part uses.
In this same way you should be able to exclude external (or guest) accounts with a query exclusion like:
-accountname:EXT (or in our case we tag all external users as "(EXTERNAL)" so our exclusion is
Bottom line, create a query that only selects the accounts you wish to exclude and then negate it.