No deleting service in ITSI does not affect the entities. In your case I would:
* identify what filters are used to filter entities to the services you are to delete.
* Use the entity management view to verify if these entities used in any other services? If yes, do you really want to delete them? If no select and delete them from the entity management UI. Preferably before deleting the service so you can verify in the service that the entities now are gone.
Also note that if these entities and or services are created by a scheduled job, they might reappear when those jobs are run again.