Wikipedia:Heckler's veto

The term heckler's veto, in the sense that is significant to Wikipedia discussions, occurs where one party tries to present a proposal, and an opponent or a small group of opponents of this proposal attempt to swamp the discussion with their repeated objections, thereby preventing a consensus-based determination from being made at all.

Example
Editor A: I propose that we change Policy X to Policy Y.

Editor B: Oppose (followed by a 500 word essay on why Policy Y will inevitably be rejected by the community as a bad idea, and a demand that the proposer withdraw the proposal immediately, or that the discussion be moved to a little-watched page where no discussion is likely, or that an administrator speedily close the proposal before a discussion can take place).

Editor C: Support. I think Policy Y is a fine idea.
 * Editor B: (posts another 500 word essay basically reiterating the objections raised in the first 500 word essay, and re-asserts his demand that the proposal be withdrawn or closed without further discussion).

Editor D: Support. I agree that Policy Y is better.
 * Editor B: (posts yet another 500 word essay reiterating the objections raised in the first 500 word essay).

Editor B then calls for the withdrawal of the proposal or the immediate closure of the discussion in a way that reflect's Editor B's positions, based on the objections previously stated and reiterated several times. Editor B may try to turn the discussion into a discussion about whether the discussion should be closed and the proposal deemed to have failed, based on those objections.

Symptoms
Editor B may be prone to mixing personal attacks into his objections, particularly questioning the motives of the proposer and supporters of the proposal. Editor B will certainly continue beating the horse well after it has putrified, and is likely to attempt to recruit like-minded editors to do the same. Editor B is also likely to use a challenging tone and aggressive terminology, making blanket declarations that the undesired outcome will not be allowed to come about, and that the community is against it, irrespective of any opinions actually expressed.

Note that Editor B is not merely trolling; he probably believes in the correctness of his position quite fervently, almost religiously. Note also that Editor B's key strategy is to try and prevent the discussion itself from going forward. Editor B may well realize that the community is likely to support the idea, and wishes to prevent the idea from even being presented for consideration.

Responses
For administrators dealing with this kind of activity, it is important to remember that decisions are made based on consensus. A single editor or faction of editors does not get to outweigh the greater consensus of the community merely because they make the largest number of posts, or rephrase their arguments in the largest number of ways. Furthermore, it is important to remember that any editor may propose such things as a deletion, a page move, or a discussion on a point of policy, and these discussions must be allowed to proceed for at least a reasonable amount of time. Opinions can change even in the course of a discussion. Outside of a clear WP:SNOW situation, the discussion should be allowed to proceed for the usual period of time for discussions of that type to proceed.