Can I throw out my flatmate's alcohol?
Why would you want to throw away their alcohol?
This flatmate of mine ruins my alcohol and they never buy me booze.
I get angry with my roommate too, sometimes, but it's wrong for you too throw something that does not belong to you. If you think that they are ruining your alcohol, then you should speak sincerely and politely on why do they do that, instead of retaliating against them. You being toxic in your relationship with them, maybe very costly and unhealthy for the both of you.
What the hell, why are you trying to lecture me about this? I don't care what you have to say.
You may not like what I have to say but that doesn't change the fact that their alcohol belongs to them. Instead of throwing away their alcohol why not talk to them about what they're doing in order to work things out? That way you two won't make your living arrangement uncomfortable for you both.