How could I automatically delete accounts with a given criteria?
You may not be able to stop the email being sent. But you can set up a rule to block the user to trigger on the event 'Before saving user account'.

You will require two conditions. First use the condiiton 'Entity has field' and check for the 'Reasons to join' field. This will make the field visible to subsequent conditions. The do a 'Data comparison' on account-unchanged:profile-main:user:field-reasons-to-join and check for your two charcter text. If the conditions are met use an action to 'Block a user' using account-unchanged:profile-main:user.

EDIT: Since your users are blocked pending admin approval, perhaps you should be looking at the Field validation module. With it you can specify a minimum length for the field (making sure you let your users know in the guidelines that the minimum length is required). You could also set up a field hidden with your css which you would require to be empty. Bots would likely fill it in with something.

You should also have a look at the Spambot module which uses the api at stopforumspam.com to catch known spammers. While it is not perfect (nothing is) it will certainly cut down on the number of bots that get through.