A government organization controls all guided trekking on the mountain. If you hire a guide he/she must be a member of this organization. From what I have read some of its members can become a little ‘old school Sicilian’ with those wishing to make the hike sans guide. Apparently, they can show up at your hotel, wait by the trail to see if you are guideless, or even threaten people who refuse to hire a guide. I am not sure what they threaten you with as causing bodily injury would ensure no need for their services.