We do not maintain a list of merchants who dont allow this. This is should be dealt with by merchants on an affiliate by affiliate basis. The network stance on bidding on brandnames and keywords is that it is allowed unless you are specifically asked not to by a merchant.
No merchant has expressed a position on this through our network. Though I know there are merchants on our network who have voiced objections to this on programmes they run on other networks.
My advice would be twofold: If a merchant has expressed a desire for thier affiliates to not use this method of advertising for them via another network, then that sentiment can be carried across to awin. For instance, if a merchant doesn't allow PPC on its brandnames on DGM2,
TD or CJ (or others), then they probably dont want you doing so on Awin.
Secondly, if the company has a registered trademark, they probably will not want you using it for PPC purporses, however some merchants may not have a problem with this, so as before, it should be dealt with by the merchant.
If you need any clarification on any of this please ask