I have to take back my statement from above - I just stumbled upon a way to indeed clear the log!
You can set the memory-lines (or disk-lines) parameter to "1" (0 isn't valid) and then back to the value you want to have (like the default "100") - this will clear the log just leaving the first line - which will then always read "log action changed by admin"...
So:
/system logging action set [/system logging action find name="memory"] memory-lines=1
/system logging action set [/system logging action find name="memory"] memory-lines=100
to clear the memory syslog content.
You could combine this with the e-mail script Nexon posted. But no more need to reboot the machine to get rid of the memory log file...
Best regards,
Christian Meis