This thread will probably go down in PpRuNe history
Case in point here - Centreline. They carry discrete branding on some of the aircraft and Phil is one of the straightest business people in the world and actively refers clients back to brokers.
If you will only put your passengers on aircraft that carry any branding I think it's a very short sighted view on the market - indeed as unbranded operators are very much in the minority in Europe you are almost putting yourself at a higher risk of loosing your client to another operator / broker by virtue of the fact that they will probably identify another cheaper source.
Perhaps you should consider the operators that have a fleet of aircraft suitable to your clients mission and can offer good customer support and try to forge a good working relationship rather than the somewhat blinkered view you currently have? Good clients will come back to you and tell you if they have been solicited directly, don't underestimate the honesty of a good client.
Interestingly, this view seems to be completely at odds with the bigger players in the market place - I have never heard Air Partner, Ocean Sky, TAG, Bookajet, Chapman Freeborn etc have an issue.
Woe betide you if you ever need to book something with more than 30 seats coming from an airline...