Actually - I have not had a supplier block a review .. yet!
I have had suppliers (UK sole distributors) ask me to write a review, of an absolute stinker of a review of a USA made multitool, and insisted that every damning word was published .. just to show the manufacturer why they refused to distribute the item against the manufacturers wishes.
Other items I have had that were total rubbish I have refused to review in full but have returned to the supplier with notes on its failings. Some "improved" items have then been resubmitted by them for review. Sometimes, after review the manufacturer requests the return of review samples ... though not many do this as most reviewers see the review sample as their "fee".
I know of many gear reviewers who have huge gear collections and others who sell on the used review samples of items they have covered - the second hand value of most used kit is pretty low! Others pass used review kit on to charities/friends/relatives.
I have had the experience of folk coming to a of wearing gear that I have donated to my local charity shop
Most of the kit I review for BcUK has been acquired for review samples to be written up for magazines I freelance for - such as Gun Mart. I get to pick and chose most the kit I get for review so rarely get any duff stuff, having looked it over at trade shows first.
Interestingly I did get told by the PR of one famous Scandinavian clothing and gear company that they would not send me one of a NUMBER of their rucksacks as it was not built for outdoor use, just as a "lifestyle look" bit of gear!