The short answer
Say 'la, shukran' (no, thank you) once, with a smile, and keep walking. Don't stop to explain. Saying you are going back to your riad, or that you already have a guide, ends most conversations. Calm and polite works far better than annoyed, and you don't owe anyone a longer reply.
In more detail
Selling in the souks is persistent, and a long answer, friendly or sharp, tends to encourage rather than end it. A brief 'la, shukran', brief eye contact and a purposeful walk do the job, even when you are completely lost.
Some approaches come dressed as help. Someone may tell you a street is closed, that the square is 'only for Muslims', or that the tanneries are only open today, then offer to show you the way. Streets in the medina don't close for tourists. Smile, say no thank you, and carry on in the direction you were going.
The hassle is mostly persistence, not danger, and it fades in quieter quarters and early in the day. If anyone turns aggressive, walk towards the tourist police, who patrol Jemaa el-Fnaa.
Good to know
- La, shukran: no, thank you.
- Safi: enough, that's it.
- Ask shopkeepers for directions, not people who approach you.
- A licensed guide on day one stops most approaches.