I've decided that I don't want my spouse going to conventions.
Oh really? What's the reason you don't want them going?
There will always be some kind of asshole there.
It's normal to worry about your spouse's safety when they go to large group settings, but you can't dictate what your spouse does. Your spouse is their own person with their own interests, and if you try to tell them what to do then they're likely to be angry and resentful toward you. Express your concerns to them in a kind and loving way, but accept that they're going to do what they want.
You might be right, but I really don't want them going.
I understand, but just because you feel that way doesn't mean they have to do what you want. I'm sure they'll take your feelings into account when they decide whether or not to go.