Clients showing up late...

Ahhh this old chestnut.

TRUST me from experience, you have to be brutal or you will burn out and hate your life.

I seem to have a lot of bands turn up early these day's, so I tend to be at the studio at least 30 mins to an hour before the session starts, good time to put a brew on and turn the computer on, etc etc.

Valid points already made I'll reiterate.

If the band shows up late, that's tough shit for them; The day starts and ends at the hours we agreed upon. However there is usually something you can be getting on with whilst you're waiting; A lot which I prefer doing without a band breathing down my neck... like setting up the session on Pro-Tools/(insert DAW name Here).
Making an input list of the mic's / pre's/ Patching you're going to use on that day (super handy when recording drums)
Get out mics and put on stands and plug in to wall box and leave in a space into which to put drums/bass/guitar/kazoo.

Get on with any editing that needs doing.
Start organising your session and bouncing Triggers and soft synths.
etc etc etc.

Can't say I've ever found myself at a lose end!
 
The easiest thing to do, as far as I'm concerned, is to just set things straight up front.

the client calls on wednesday say, books 12-6 slot on a Saturday, recording one track. I'd explain to the group(my "contract") the way things work in my place to make the flow and efficiency work.

Everyone pays up front. If I don't have at least 70% of booked time that day in hand, no one goes In The studio. However, burn them a copy of the project folder after the session, so my end product is a mix and the folder of all the raws

That says they get charged the slot they book. It's time they have reserved that another band could have made use of. 10-15 minutes late is fine every so often and it's fairly easy to tell who is a habitual offender.
 
in the time of smartphones, send them a fb message, e-mail, sms or whatever exactly at the time
the contract says you start that says "you're paying every minute from now on".
 
Why does it matter? I set a "closing time" of sorts. They then assume they can't go past that time. I then proceed to charge them for the hours that they've missed, because I didn't make the plans to sit around. Done.
 
the thing that worked for my band when recording in a studio was that Tim the engineer said no matter what he would kick us out at 8pm so he could backup and edit in peace. It meant that if we got to the studio on time we got the full day and if we were late we were pissing our money away.He charged by the day and we had to pay for the day in advance too.