I know this is a simple question, but I'd appreciate knowing what "best practise" should be. We've had 4 purchase invoices created and have been asked to cancel them. We have a company policy that say we should not delete transactions, so what is the best practise method to cancel their impact and not show on the supplier record.

Vendor credit

That's what I thought, but last time I suggested this they complained as they didn't have a credit note from the supplier but have just entered the PI in error.

You could technically do a JE and apply the JE to the purchase invoices to close them, but if this is a regular occurrence, I wouldn't have that be your standard process. Applying JEs to stuff can be a little flaky. Using a vendor credit and just putting something in the memo like "billed in error" or some standardized thing makes sense to me as best practice.