I was recently in a discussion with a fellow instructor about how annoying it was for people to be reading ahead while he was trying to teach…
How do you keep the attention of student who like to read ahead of the presentation?
Although there are many reasons why people start reading ahead of the presenter, but the most obvious reason is because the materials are there. In my experience, if you give people the materials you are presenting, many will read ahead… especially if you present the same material they are reading. My recommendation is to avoid distributing directly-related materials to the participants until after the material is presented. This allows them to focus on your presentation, and then refresh themselves with the handout materials.
I am not adverse to providing materials to the students ahead of time, especially if it is a reference such as a worksheet of manual. You just want to avoid giving a handout which contains the exact information you intent to teach.