Hi,
If you want to replicate only customers from CRM to ECC, you can set a filter in the subscription on IS_CUSTOMER = "X" for example. That will stop replication of contact persons, organization units, employees, etc.
See following oss note: 1834681 - How to set filter for BP upload from CRM to ECC
But for something more complex, you might need to implement a function module and add it to table CRMC_BUT_CALL_FU. Kindly check following oss note for an example:
638628 - No contact person upload into R/3: Example for exit users
Best regards,
Nicolas.