[KaVir]

Your mud isn't a legal entity. If you want to prevent people from removing their copyrighted work from your mud, you need to persuade them to grant you a non-revokable licence to use their work.

If you also wish to control how they use their work elsewhere, you'd need to get them to sign over their rights, or grant you an exclusive licence. However this also means that if your mud dies, it takes their work with it, so you may find that many experienced builders are unwilling to agree to such terms - particularly if they've been burnt before.