As I understand it, this issue affects Exchange transaction logs. You absolutely should not 'limit' these logs in any way.
Transaction logs are committed to the exchange database and flushed after an exchange-aware backup. If you notice transaction logs growing, you have two options - move the logs to a drive that's big enough to hold a typical day's log growth under these circumstances (I'm assuming you carry out a daily exchange backup there) or increase the frequency of the backups.
I'm still looking into ways of dealing with the problem on Exchange 2007 myself, but the issue needs to be fixed by addressing the devices causing the problem rather than limiting the database files on the server
Depending on how your firewall works, you may be able to block access requests from 6.1 devices based on user-agent string (think it's
Apple-iPhone3C1/1002.144 for 6.1 devices)