There are a lot of layers in your question – perhaps more than you’re even aware of.
“My husband of 10 years wants to transition to a female.” This might be difficult for you to read, but you don’t have a husband, you have a wife. It’s ok, that doesn’t make you a lesbian.
“How do I stop him?” Do you love your spouse? If yes, why would you want for them to suffer – to make you happy? That’s not loving. If no, why are you together?
“Him”. Nope. Her. If she’s wanting to transition her body to female, she’s a her. She’s a woman.
Ok, perhaps I’ve been a little harsh here. I’m sure you’re surprised, shocked, blindsided, hurt, angry and/or devastated. Those are all valid emotions. Feel them – they exist, and you cannot ignore them. But consider getting some help with them. Friends are great, but unless you have friends who have had spouses transition with them, most friends aren’t going to be able to understand what you’re going through. They’ll try to give you well-meant feedback and advice, but frequently, what they say will end up hurting you and/or your spouse. Try to find support groups for spouses of trans people. Also, find a therapist to work with you – just you. Trust me, you’re gonna need it. It wouldn’t hurt to find a marriage counselor with experience with transgender spouses – my wife and I have one who is absolutely amazing!
Your spouse is vulnerable right now. She has just revealed one of her deepest darkest secrets to you – one she might not have even been consciously aware of until recently. She really needs support and affirmation. If you can provide that, that is fantastic – she needs it really badly! But if you cannot, please please please don’t come at her with anger and accusations! Explain where you are and what you’re capable (or not capable) of. Remember that you two love each other (you do, right?) and that both of you were good with each other before. If you can’t be her spouse, at least be her friend.