REX are now known to be using giveawayhopper.com to give keys away.
The problem is that there are legitimate non-REX giveaways, so... Instead of blacklisting the site outright - REVIEW IT. Check the actions. If they require you to join groups relating to them, and/or click on shortened links (especially that particular one which pays the user for linking with them), then it is associated with them.
@StatiG - is this enough warning for you? *rolls eyes*
Update: Due to overwhelming demand our servers are struggling to keep up and there may be a delay in sending out keys. You will receive your key but it might take up to a week to mail it to you. We apologise for any inconveneince. [sic]