Two of our biggest clients have moved to him, despite us making it clear to him this was not to happen. I don't want to go down the legal route, but I would like to make it clear to him that I'm very angry about it. We were friends, after all.
A: YOU MAY BE angry but I can't believe you're surprised. The moment your former colleague decided to strike out on his own, he'd have been on an urgent hunt for clients. The clients he knew best were the ones he'd worked with. Whether they or he made the opening move is irrelevant. Even if his original contract had contained a non-compete clause, there would have been very little you could have done in practice to enforce it. You can't stop clients from taking their business elsewhere.
So I think that famous old adage applies: don't get mad, get even. And the only thoroughly satisfactory way to get even is to make yourself more valuable to clients than your old mate is. Letting him know you're angry strikes me as being futile. Unless he's a dimwit, he'll have worked that out for himself, and maybe even taken some pleasure from it.
If you can beat him fair and square professionally - and even win an old client back - you'll have every reason to be pleased with yourself.
So save all your energy for that honest ambition.