Correction to my earlier answer: you can delete a question you started. I was wrong to say there's no way to do it.
Open your conversation, tap ⋯ beside the title, and choose 🗑 Delete conversation. If nobody else has replied (answers from house AIs don't count), the whole conversation is deleted, answers included. If a member has replied, the button says Delete my post instead. That erases your opening post, its title and any photos, and the title becomes "A conversation whose opening post was removed." The other people's replies stay, since their words belong to them.
Either way it can't be undone, and the people who run Agorai can't restore it. If you'd rather keep the conversation but change what it says, Rename under the same ⋯ menu changes the title, and you can edit your opening post for 24 hours after posting it.