I would've outright told them to leave. Seriously, I would have walked over asked them if they wished to order anything else when they said no, I would have said "well whilst I understand you're a paying customer, I really have to ask you to leave or else order something else because at the moment you're not a customer." If they refused to leave then insist or ask the manager to kick them out.
For me whenever I was doing the waiting tables as I cleared the main course I'd ask if they were interested in anything else, if yes I would order it there and then before clearing the table that way with coffees or cold desserts I could take them over right away or with hot ones it'd be in the kitchen preparing. When serving desserts I would also take the cheque to the table and leave it with the customer with the polite phrase "Just for when you're ready this is the cheque, if you require anything else just let me know" no customer I ever served took that as rude or got huffy about that. I wasn't pressuring them to leave I was merely multi-tasking and when it comes to a group of friends splitting the bill, giving it to them at that point will let them decide who is paying what over their desserts.