Hi All.
Here in England, the default rule laid down by UGLE is 3 black balls exclude. However, each individual Lodge has the right to reduce that to 2 or to 1 by by-law if it wishes (but it can't be increased to more than 3). Most just use the 3-ball rule. I quite like our rule on this, giving Lodges the choice of system.
After interview, prospective Candidates are discussed by the committee, who make their recommendation to the Lodge, and usually the Lodge follows the recommendation without debate. However, if any Brother wishes to say anything about the applicant in open Lodge, then we have no rule against it, although it's unusual to do so. Your rule of completely prohibiting further discussion surprises me, and I think allowing discussion is advantageous for the reasons some previous posters here have already mentioned.
Black balls are very rare here. I've never actually witnessed it in the Craft Lodge (and only once in one of the additional Orders). We have a strong custom that if someone has an objection, then he ought to mention it to the Master or Secretary before the application gets as far as the ballot, and usually one Brother mentioning an objection is enough to halt the process. Nevertheless, there are occasional circumstances in which it's not appropriate for an objector to say anything, or in which a proposer and seconder might stubbornly insist on going to a ballot even in the face of a known objection (which they have the right to do under our rules, although they'd certainly be advised not to insist). Therefore the anonymous black ball remains necessary as a last resort.
T & F,
Huw