A question, can i disipline a person for using a smart phone during working hours?

An example, on Saturday morning i have some staff working on site, from 7 am when works starts until 1 pm the phone goes non stop. beep beep, beep beep from messages, the staff member goes to the toilet or out to the vehicle and takes longer than they should to return, because they reply to the messages. I dont have a problem with staff members using the phones while we are travelling to site or during lunch time, but it is getting out of hand with the social networking during working hours and texting/messaging. The other day i was talking to one of my staff members showing him what to do and his phone beeped, so he takes it out while i am showing him what to do and replies while half listening to what i am showing him, he then doesnt perform the task as requested. The worse part is he is learning the job.

Today again i ask him to do something, his phone goes all day long, he tells me he is finished what he had to do and asks if he can leave, thinking the job is done correctly, i let him go, finish what i am doing and go finish my part of the job to find his part was done incorrectly. I believe he is not perfoming his tasks correctly because he is being distracted by his phone and the messages.

Can i ban cellphones from site? I will provide a cheapie like i use with prepaid sim card in case anyone needs to get hold of me or if i need to contact someone on site?

Another issue i have on site is people are using their phones to listen to music, so they plug earphones and cant hear you when working. The other day i displined someone for doing something dangerous, he looked at me and thought he was just ignoring me, but it turns out he just couldnt hear me because of the music blarring in his ears.

So my reasoning for banning cellphones on site will be that they pose a threat to my safety and other members working on site. Do you think i would have a leg to stand on if i was taken to the CCMA.

I am also tired of begging people to work on weekends, so i am also going to add into my employment contract that 2 weekends a month is required from all staff members, can i do it?