I'm assuming this is a university level question.
The best way to approach this is on day one, where course standards are set. Trying to institute something in the middle of the semester can only start arguments ("It's not in the syllabus I can't do this!" or "But we've been doing this all semester to take notes!"), but if necessary, give them some forewarning to the new policy.
I find that, particularly in math courses, it is difficult to take notes on a laptop unless the instructor provides the slides (and if those should be used in a class is another question entirely). Additionally, many classes are now going online for their homework, so students will use class time to work on homework. There are a few questions here. Is my course easy to take lecture notes on a computer with? Do I want students working on homework in class? What do I do if students are using technology inappropriately?
This answers depends on the the structure of the course. In all cases, I will make an announcement on the first day of class stating my policy (no laptops, limited laptops, etc) and what I will do if someone breaks the policy (ask to put it away, stop class, ask to leave, etc). You need to set clear boundaries on distractions to the students and have them know the consequences. They know that classes are for learning and that you are there to foster that. A reasonable student, when asked to stop playing Quake for example, will stop. If you have TA's, I would ask if they would sit in the back and inform you if someone is being distracting. Else I would occasionally move around the room during lecture and to pay attention to where students eyes are looking. It should be fairly obvious when someone isn't working.
That said, the level of technology is course dependent. As I am not too familiar with power point style lectures, I would leave that up to your discretion and decide if you post the lectures online before or not to have laptops. Personally in math, I find that the use of hand written lectures (either on a projector or chalk board) is the most used. In these, I find that it is near impossible for students to type up notes at a comparable rate to your lectures. In this case, I would put in the syllabus that laptops and other devises are not allowed in lecture and that I expect students to copy notes into a notebook. If students pulls out a laptop, I would just politely ask them to put it away and remind them of the policy. Now some days you can make exceptions (review days, etc), but always let them know at least a lecture in advance. With this, I believe that homework should be done outside of class and is no excuse for a laptop. If I do a problem in class, students should just copy it to the notebook.
Technology in class can be beneficial, but I find it is more engaging than the instructor most of the time (particularly if they are reading off slides). To fight off student boredom, keep them engaged and interacting with you. Ask them questions, poll them for answers, and get them to work together will help push off their want of technology. I've seen an increased use of flipped classrooms and clickers to poll students and keep them engaged. You can't win against phones (just ask for them to put them away if seen), but if they are enjoying class, they are less likely to check them.
As you say you are more or less stuck to using slides, why not see about getting a tablet and use computer written notes? Your IT department might have some for loan for use in lectures. I find that using slides is the leading cause of distractions if you let them use laptops.