Yes I have an alcohol and drugs policy, I won't/don't want to see anyone who is drinking, drunk or under the influence of drugs.
I have been tolerantish in the past and have had some poor experiences, which I don't want to have to deal with.
Having become more and more explicit about this with new clients, some of whom have tried to get a bit clever (think necking pints and shorts in the first ten minutes or just before the start) I will now walk away or refuse to continue with the booking (and have recently done exactly that).
I've also turned up to hungover clients who have it oozing out of their pores, sweating, ill, and affecting performance (usually they end up berating themselves) so I often add this into my pre-booking-acceptance spiel.
I do have regulars who will have one beer just before they arrive, or a glass of wine but most won't. This might be because they know a bit more about me(I'm teetotal because of a reaction I have if I consume alcohol), so avoid doing this out of sheer respect.