@Zoë20I've had good luck in getting guests to cancel the booking themselves and having the third party create their own profile and then make the booking. As @Ana1136 has pointed out, neither you, nor the guests will have support through Airbnb if you allow a third party booking to stand.
It's also worth noting that even though you are "supposed" to have 3 penalty-free cancellations with Instant Book bookings, many hosts are seeing themselves removed from Instant Book after cancelling with cause for a period of about a month. This is why I do my best to get the original 3rd party to cancel themselves.
I simply let them know that 3rd party booking are in contravention to Airbnb's Terms of Service, Section 14.1(6), and that while I would be overjoyed to host XX, they must book with their own profile, as stated in my listing rules. I then ladvise them to cancel the booking right away before the 48 hour full refund period expires, and to co-ordinate timing with the actual guests so that they don't miss out on the nights they want. Everyone wins - they get all their money back, the real guest has full support through Airbnb and you don't expose yourself to financial loss through no support and/or losing instant book for a period of time.
Fingers crossed, this has worked for me each time the issue has come up - except for once where the third party didn't end up booking. I figured that was fine by me.