How effectively could clerics, druids, paladins, inquisitors, and other socially acceptable casters keep the relatively small population of the land's wizards and sorcerers in check?
Pretty darn effectively provided that they have the cooperation of the full society.
While wizards can be neutralized by taking away their scrolls, spellbooks, and magic items, you can't simply confiscate a sorcerer's bloodline.
This is one of several reasons why Wizards generally enjoy greater social standing than sorcerers. I'd say the majority of my map tolerates Wizards, though with a variaty of restrictions - you have to register your status and rank, you have to announce yourself when entering a city, many spells are illegal, etc. But sorcerers pretty much every where are officially classified as malevolent entities, and are subject to the same treatment in theory that lycanthropes, undead, and fiends from the lower planes are subject to.
To check their propagation you would have to check their population, and good-aligned characters would rightly object to most if not all the most efficient or effective methods of pursuing this objective.
Would they though? Wouldn't this first require the good-aligned character to recognize the sorcerer as a fellow being with the same rights as himself, rather than as a fiend clothed in human form - and therefore doubly vile for being both evil and presenting itself as something it is not? And keep in mind, for my particular take on the Sorcerer class, this belief about sorcerers is particularly easy to sustain - most of them are in fact not fully human. In fact, for several bloodlines the character has the option to mutate as they gain in power and they become visibly inhuman. (My sorcerer can be thought of as being inspired by the mutants from X-Men, and have much the same issues.) It might take a particularly enlightened and wise good person to realize that sorcerers aren't all cut from the same cloth. And in any event, who says that the majority of inhabitants of society are good? I tend to estimate that 80% of any population is simply neutral - indifferent, mildly self-interested, reactive rather than proactive, unprincipled, changable, ecletic, apathetic, weak, thoughtless, etc. Few adhere to any rigorous moral code. Divide the remaining 20% among the other 8 alignments, you and you end up with maybe 1 person in 30 with the moral fortitude to even question 'common sense' and do something about it. IMO, atrocities occur in real human history seldom because a society is filled with evil persons in much greater numbers than found in another.
In my opinion, this is one of the more important themes of my world - recognizing that you are part of the few not merely in power but potentially in belief and deciding what to do about that.
Above a certain threshold, the population of genetic casters becomes unmanageable by moral methods...
Burning at the stake any presumptive sorcerer, regardless of age, for example? One of my adventures revolved around the ancient custom in a rural region of throwing newborns down wells if they were believed to possess signs of being tainted. Infanticide has been an accepted practice for much of real human history. Consider how easily it ties in with the fear that you have given birth to a real monster. Consider real human history and think about how criminals of all types were treated until very recently. It's 'obvious' that the witch panic in the real world was horrifying and immoral, because we know that the persons so treated had no real unnatural powers, but is it so obvious that it is immoral if in fact we assume that they do?
Keep in mind one of the major tropes of D&D is that high level characters and high level arcane casters can become fantastically powerful. Given that power is known to corrupt, and the extreme almost incomprehensible vulnerability ordinary members of society have in the face of a high level character, just how unjustified are these sorts of preventative measures really? How many campaigns are about saving the world because some high level caster wants to break or remake it? I think it would be a mistake to trivialize the very troubling problem of ends versus means when it comes to a society protecting itself from the use of magic. I consider that a player hasn't really considered the real horror involved in spells like charm person, animate dead, monster summoning, polymorph other, scrying, etc. etc. if they look at this problem and are quick to judge the 1st level commoners for thinking, "It's either us or them."
