I have done myself, and seen many others IL with 5 US airlines, 6 Chinese airlines etc, to increase competition and realism (not many airlines in the real world IL with more than one airline per country) we should set a limit of 1 or 2 airlines max per country?
What if one player has 4 airlines in one coutry due to reaching the maximum flight numbers? Then you want to force his alliance members to pick 1 or 2 of his airlines to cooperate and ignore the rest?
You can just put that requirement into your airline description or negotiate exclusivity via direct messages with your prospected partners.
If you can agree on the terms with them, fine. If not, then you can still decide whether to take the IL anyway or reject it. It’s a free economy, I don’t see the problem here to be honest…