You can delete one, more or all transactions in an account on your Mac.

Delete one transaction: 
Select the account in the list od your accounts > click on the transaction you want to delete > press the delete key to delete this transaction

Delete some transactions: 
Select the account in the list od your accounts > mark the transactions you want to delete while holding the cmd-key > press the delete key to delete these transactions

Delete all transactions from an account: 
Select the account in the list od your accounts > mark all transactions with cmd+a > press the delete key to delete the transactions