Show that you enjoyed the exchange with the recruiter
In sum for refusing a job, you must be sincere and thank your interviewer in order not to offend him. For example you can say:
"Thank you very much for your offer of Lead Developer. Thank you for taking the time to consider my application and answering all my questions about the company and the position. "
"I thank you again for the interview last week, I enjoyed meeting with the team and visiting your offices. I enjoyed our discussions on the post Growth Hacker and thank you for this generous offer. "