Modify emails posted in messages

Is there a simple way to transform email addresses (eg: user@domain.com) posted in mesages to something like: user[at]domain.com or user [at] domain.com? This is basically to prevent spam bots from harvesting these emails.

There is no simple way. You need to introduce new BB code either invent something automatic which won't be perfect anyway. However, it's up to users who are posting their email addresses... if they want so, why not to allow them?.. ;-)