disk_total_space (string directory)
would return the bytes used for a directory.
You could run up a cron job (daily) that meanders quickly through
the list of named dirs and tests the byte usage, mailing you when
one of them goes above 10meg.
Fiddling about with ftp permissions (ie blocking an account that
went over the ten) would be a tad harder to say the least.
for scriptlets that scan sub directories too.