The vast majority of instances are one man shows where the admin also has root access to the server the instance is running on. Aside from that I don't think th

The vast majority of instances are one man shows where the admin also has root access to the server the instance is running on. Aside from that I don't think there's anything on the Mastodon webapp itself that lets you post as a user. It's mostly limited to blocking/hiding instances and users.