API imports and Wallet Imports will always import all transactions that are not listed in your account.
If you delete a transaction it will be re-imported again.
You can either set the value of the transaction to oero (thus the TX stays in the system, but the value is nothing) or you can alter the API:
- set the import from "automatically" to "manually" (only for API imports)
- delete the API key or the Coin Address
- set a "start date" for the import on the import page