Hello Christophe
Authorize.net is a different payment gateway, not related to PayPal. So if you only need to use PayPal, just forget about Authorize.net (I see that Authorize.net is mostly uses by US Customers)
So in this case, just go to Events Booking -> Payment Plugins, unpublish that os_authnet payment plugin. If you only use PayPal, then you should unpublish other payments method as well
Regards,
Tuan