How do review websites ensure the quality of their reviews in terms of spelling, grammar and imperfect/incomplete sentences?

With sites as big as Yelp or some of these other ones, manually moderating every single comment is not practical Therefore, there are several things a site can build into their website to help make this process more automated. Here are some examples: Automatically filter comments through a list of terms that are not allowed (cursing, racial slurs, etc.) Require a minimum number of characters or words to prevent low value comments Add an option on the comments themselves to allow other users to flag a comment as spam or as "inappropriate" (this helps lower the moderation load on staff) Automatically remove comments when they are posted in rapid succession or disallow rapid comment posting all together (ex. allowing 1 comment every 2 minutes) Automatically removing or at least flagging comments that post the same link over and over again in posts Companies such as those you mentioned usually reserve the right to remove any comment or user for any reason which allows them to ensure that they maintain control of their website.