So this is just an idea:
Why don't they make it so that if a player is premade with a certain player, the honour option isn't available to them. Most honour trades are happening between friends right? Premades etc. I think this would minimize honour trading and Riot should really consider it. Players begging for honours from other players in chat etc should just get reported (spamming).
By honour option not being available, I only mean it in between the two premades.So example:
Player A and Player B are premade. They do not have the honour option after games for each other. Player A can't honour Player B, and Player B can't honour A.
However, Player C, D and E, are not premade with player A and B. Player A and B can both give honours to Players C, D and E and the opposing team.