The only question I have for the OP is why does she want clients who do not respect her time or boundaries?  Your clients are paying for a specific period of time.  You have every right to ensure they honour this, after all, that's what was agreed in order for you to accept the booking.  Decent clients will understand this.  Anyone who accuses you of being a clockwatcher is only pissed because they know they can't overstay/you've called them out on their disrespectful behaviour.

Decline any further bookings with them and if they press you as to why, tell them.  The decent ones will book (and stick to) a specific time.  The others whom are only interested in getting extra time will go elsewhere. 

Sound like you need to work on your boundaries OP, you're running a business not a charity. I believe that if your boundaries were solid and you were much firmer with these guys you would be making more money. They're taking the piss.

Oddly enough, with new clients, I don’t tend to worry as I much if I am strict. It’s with ones I’ve seen a while. I think I am going to completely stop “extra” time. It’s probably blurring things .

I've had this problem in the past. So much so that I felt I was being walked all over and had to take a break. I returned with boundaries. I will never be comfortable with setting alarms or telling people to leave however I do now get up myself and say that I had better start getting ready (have a shower, pop out, have lunch etc) and start busying myself as I talk to clients. They follow my lead and do so too. Good regulars will respect your time and will adapt to your new found boundaries.