One of my former Mistresses had some pretty extensive experiences with extreme sexual conditioning and control. She was very active in the lifestyle and attended many group functions and encountered many couples where the Dom had developed such a high level of control over the sub's sexual response that they could basically get turned on or off upon command. In the most extreme cases the sub could either orgasm almost immediately in response to a particular command or couldn't orgasm without a command.
This was fairly early in my entry into the lifestyle and I found (and still find) this idea to be very intriguing and quite a turn on. e.g. If she said "get hard" you'd automatically have an erection. If she said "go limp" you'd lose an erection. If she said "cum" you wouldn't be able to hold back an ejaculation even if only minimal stimulus was applied. If she didn't say "cum" you couldn't ejaculate.
Unfortunately things didn't last long enough to ever go down this path but the idea has always remained in the back of my mind. Over the years I've found there's a few ways that are able to manipulate the male sexual response, some of which are physical and others are more emotional/mental. I find some of these options to be appealing while others are not. I don't consider chastity to be a direct part of this (although it can be used to assist in conditioning) since it only controls the ability to get an erection, not the attempt at getting an erection.
The easiest method is through the use of a device. A KTB comes to mind as probably the most prominent device of this type to condition a male to not get hard. While effective it's probably best used in situations where the Domme doesn't want the sub to think of her in a sexual way. Even if he is interacting with her sexually, he's not supposed to respond with any type of arousal. A KTB definitely falls into the realm of negative reinforcement.
More intricate conditioning methods often take a lot of time to establish but at the same time, if they happen regularly over a long enough period of time the conditioning can firmly become a part of the sub's sexuality. Consistent (or even ritualized) negative reinforcement can breed a subconscious association with the action leading to the negative treatment. I believe the key here is that the negative reinforcement has to go beyond what will yield any enjoyment for the sub. This may be difficult in some cases, especially subs that are intense masochists or crave complete degradation. With some experimentation a Domme can probably find some negative act (or intensity of a negative act) that doesn't elicit a sexual response from the sub. With enough time and negative conditioning a sub may stop responding sexually to something that once turned them on or find it much more difficult to get aroused by something in particular or in the most extreme cases find it difficult to get an erection without permission.
Using positive reinforcement to condition a positive sexual association is a bit more straight forward but it can be quite a bit more interesting depending upon how it is used. In its easiest practical use, ordering a sub to partake in an activity the Domme enjoys but he dislikes and granting him a sexual reward afterward will eventually develop a positive sexual response in the sub towards that activity. But what if this is taken to the extreme?
Allowing a sub to ejaculate only under particular circumstances or during a specific act, in a certain environment, or under certain types of stimuli can, in essence, develop a fetish in the sub. Since fetishes are believed to be developed through excessive masturbation while fantasizing about a specific object/situation, limiting a sub's ejaculation only when certain criteria are present (and possibly emphasized) is pretty much the same thing.
With a bit of imagination it seems like a rather devious tool to be able to cause a sub to get off over something they initially had no sexual interest in.