to bad for TNX.. i used TNX.. and the keyword that I've submitted to them was also gone in SERPs. huhuhu
no they hvn't used robots.txt, have a look at their file. http://www.tnx.net/robots.txt may be they have deindexed their site using google webmasters tools..
tnx.net is still in google so it has not been banned. Do a "site:tnx.net" and you will see several 1000s pages indexed. http://forum.tnx.net/ has a PR of 3.