In Google Analytics, you can define so-called reference exclusions in tracking, for example to track a payment page such as paypal.com not as a reference or home page. It isn't possible with Matomo. Since many of these had already been requested in the forum:
It would be very nice if you could implement that.
@Lamy-ux it is possible to exclude some websites from being marked as "Referrers" to your visit.
To do this, simply define these website URLs as "Alias URLs" on your website in Matomo.
You can do this by editing the websites in Administration > Websites > Edit, and entering your URL of your payment gateway under Alias URLs.