I actually like kissing and oral on me generally, it's just those odd clients whose style of doing it I don't like: tonsil lickers, dribbling saliva, etc these types have been described on this forum many times. It's these tipes that I'd like to say 'stop' or 'no' to. Or if the client is let's say, not necessarily unwashed, but has yellow teeth, or I suspect caries on his mouth.
To be honest, when someone's mouth is so fetid that you can't bear kissing him, there's nothing you can do except explain politely. It's usually the types who think putting a bit of toothpaste in their mouth suffices for oral hygiene so even if they could go and brush again (and they can't, because if they do there will likely be tiny lesions in their gums from the abrasive toothbrush which adds extra risk to any services involving their mouth) it would make no difference.
Other services can at least be offered covered, so it's not like they have to be off the menu entirely. Sure, no client is going to be pleased about having to wear a condom on his fingers if he wants to put them inside you, or to have to use a dental dam or receive covered oral, but when someone's penis or fingers are dirty there's no way in hell any person should have to be touched or penetrated in any orifice by them uncovered.
I try to offer OWO to every client because it's a service I advertise and my reviews and feedback reflect this, so I try not to fret too much about saying, "Your penis smells really odd so I'm sorry [that you are so icky] but I'll have to use a condom for oral" in the rare but very unpleasant situation that it's necessary.
At the end of the day, what's the worst that can happen? Mr. Stinky gets in a huff about his own revolting stench and storms off? In my opinion, that's far better than me having to try desperately not to vomit on him - bookings where your stomach is constantly turning are the absolute worst!! As long as you explain in a polite and friendly way the reasons for him not having access to any uncovered services, though, he really has no right to complain. All a punter has to do is A) pay upfront and B) be clean, polite and respectful. None of those things are remotely difficult so in the thankfully-not-too-common situation where a client fails those basic things, I feel much happier just showing him the door.