How can I stop someone from continuously tickling me?

You could ask them to stop if it's really bugging you and something serious. You could tickle them back if they're ticklish in any way shape or form, play back. You can pretend to not laugh and or keep a blank face so they possibly get bored with it, leave you alone possibly. Ask them why they're tickling you and if it's something stoppable it could solve that issue for you. Give them something else to do to hold their attention and keep them distracted so they're not focused on trying to tickle you. Maybe just roll with it and see if it goes away on it's own, that's always an option.