Help for REST API call to remove assigned account definition and delete employee records

First thing to research is the table which stores the association between the Person Identity and the Account definition, i'd recommend using the Manager administration client with the "View Field Definition" toggle attached to research that, that will show you the table. Then you use the REST API to CRUD that table - effectively removing the account definition that you're interested in, that's the process I would follow.

I assume you took a look at the REST API documentation, that explains the basic usage schemes of the API.

1.  Removing the assigned account definition is a little bit more complex as what you have to do depends on the way the account definition was assigned (direct, indirect via roles, requested). For each case, you either need to either remove the assignment directly (https://support.oneidentity.com/de-de/technical-documents/identity-manager/8.1.1/rest-api-reference-guide/13#TOPIC-1250873 ), identify the origin of the inheritance and remove the role membership that will end the assignment of the account definition or end the request of the account definition.
2. You just need to call delete for a single entity as described here https://support.oneidentity.com/de-de/technical-documents/identity-manager/8.1.1/rest-api-reference-guide/10#TOPIC-1250850