It is possible to create an account with a specific e-mail address just by logging into it:
However, a putative troll will not have the private keys of the former user of that account, so they can block, but not MITM or impersonate (unless talking to someone using Autocrypt’s trust-on-each-use and not DC’s out-of-band key verification).
Having multiple relays on a profile makes it hard for a troll to block you!