This is another question I got asked a dozen times already, and since a member of STM asked it in a PM again today, I thought I'd write a short post about it for all to see 
THE QUESTION: If a placement doesn't perform and I cut it, should I blacklist it in all my future campaigns too?
Thou it might sound like an easy decision to do so, its not.
Just because the traffic didn't convert with a set of creatives OR with a particular offer OR in a particular GEO ... that doesn't necessarily mean it will not convert with a different setup. Every campaign is a different beast and needs a different approach.
Having a personal blacklist for traffic sources you are using the most, is a good idea that can save a ton of money on initial tests. Just treat it with caution and don't blindly blacklist everything that didn't convert at the first shot.
The only time you should immediately put a placement on your personal blacklist is when its flooded with BOT traffic. Bots will not convert, no matter what you do. On top of that, sending too many bots to offers might raise a flag with the more suspectful of them. Even in this case, some level of caution is appropriate tho - I have seen placements that had bots only in certain GEOs - however, placements that dont mind selling you bots cannot be trusted so feel free to blacklist them.
There is one more situation when you might decide to block placements across all of your campaigns - mega sites (placements). This happens a lot in adult because there is a high concentration of high traffic websites you can target. These sites attract so much competition that its pretty much impossible to turn profit on some of them. Trying to turn positive ROI on those with standard offers and street payouts is impossible, so you might as well save some money and block them straight away. I use this tactic too, I stay away from a number of super large sites (not all) until I know I have something special to sell on those, then its time for mega scale 
Except for the above mentioned 2 examples, I would not recommend to automatically block placements from future campaigns based on poor results from prior ones.
Nice share.
What kind of rules do you follow when cutting? 3x offer payout or after X amount of visitors?