If someone is able to edit characters in your server's memory, you're already completely screwed, because they have access to your entire server. I've yet to see anyone manage this - it'd require some pretty epic hacking or them getting hold of a password.
The things you need to worry about are not in-memory edits, but weaknesses in the client during character creation and leveling. During creation, it's possible to swap out the character file created with one made clientside. You can guard against this by verifying characters that are new. You can ALWAYS tell new characters because they will have 0 xp - that's one of the things that cannot be edited for a successful swap to take place. After verification, given them a point of xp, and they won't pass through verification again. This type of verification will prevent many 2da edits, as well, where they aren't explicitly replacing one toon with another, just using altered 2das to gain access to feats, abilities, etc, they wouldn't otherwise have.
The other weakness is during levelup. For reasons beyond me, bioware chose to use clientside 2das for some things during levelup, rather than serverside. You must therefore verify on levelup as well, to ensure than haven't, for example, made a CC skill a primary skill. Such edits are a vanishingly small minority, however, and it may not be worth your trouble - they're just not a big enough advantage to appeal to the typical ex_plo%it*ers mentality.
Funky