Rule question; is it against the rules to predetermine alliances by playing with friends/ family in pub matches?

Have been getting into strings of games lately where three players allied before entering the match and proceeded to steam roll everyone.

I don't think it is specificly mentioned In the official rules.
Thought it is against the spirit of the game and many players frown upon it.

It is not against the rules to play with a friend, but it is against the rules (or at least used to be) to ally with a player before you enter a game. I've played a few games with real life friends and it's a lot of fun, but we never explicitly agree to ally before anything starts. Sometimes we end up working together, but other times we team up with strangers and work against each other. I never play rated games with friends though, as a lot of people see that as multi boxing. In your situation, that's basically a predetermined alliance, which used to be mentioned somewhere as against the rules.