No. If the buyer accepted your delivery, he/she can not cancel the order in the future. But the buyer has the ability to make a chargeback.
To date (Feb 14, 2022), Fiverr has not any declaration or clarification about chargebacks in their ToS (terms of service). However, generally, a buyer gets banned after making a chargeback. And the buyer will never be able to use his/her Fiverr account after that.
Fiverr has provided a little information (in an article) about the chargeback & seller protection but I don’t think it (seller protection) works properly.
Once a seller delivered order, the buyer has 3 days to accept or reject or ask for revisions. If the buyer doesn’t respond to the delivery, the order will be completed automatically.
And after the auto mark or auto-completion of an order, the buyer has no way to ask for revisions (officially). And even they (buyers) can not cancel the order. The only thing they can do is leave a review. See how to leave a review after the order is automatically completed.
What if a buyer cancels the order after delivery
A buyer can cancel an order after you deliver it. And there is nothing so much you can do with it. The only thing you can do is negotiate or discuss with the client. And even Fiverr customer support can not do any favor for you. Learn more about it.
A buyer can not cancel an order on Fiverr after accepting delivery. I mentioned the chargebacks just to let you know the possibilities. But you don’t need to worry about it because it’s a very rare case. If the buyer accepted your delivery, you’re in a safe situation. Good luck!