Update: for the same solution in modern search – please refer to “Hide non-personal accounts from modern Microsoft 365 search”

The following solution is for Classic Search only. 

What I did is:

User profile services -> Manage User properties: create custom property like “HideFromPeopleSearch”, boolean, do not allow users to edit value, Indexed

Client-side PowerShell script using PnP library:
Connect-PnPOnline -Url https://domain-admin.sharepoint.com
…
$nonPeople = Get-ADUser -filter … # based on what’s in your AD and how you separate people and non-people accounts
foreach($account in $nonPeople) {
  Set-PnPUserProfileProperty -Property ‘HideFromPeopleSearch’ -Value ‘True’ -Account $account.UserPrincipalName
}

SearchCenter -> Site Settings -> Search Schema: use any pre-created RefinableString managed property (e.g. RefinableString33), add mapping to crawled property people:HideFromPeopleSearch,

SearchCenter -> Site Settings -> Search Query rules: Local People Results, new Query rule, change ranked results by changing query, {searchTerms} -RefinableString33=True

should work

See also:

• m365 People Search and custom AAD and User Profiles Properties