I do not use this, but I have seen it.
http://wiki.mikrotik.com/wiki/Manual:RADIUS_Client
Mikrotik-Recv-Limit - total receive limit in bytes for the client
Mikrotik-Recv-Limit-Gigawords - 4G (2^32) bytes of total receive limit (bits 32..63, when bits 0..31 are delivered in Mikrotik-Recv-Limit)
Mikrotik-Xmit-Limit - total transmit limit in bytes for the client
Mikrotik-Xmit-Limit-Gigawords - 4G (2^32) bytes of total transmit limit (bits 32..63, when bits 0..31 are delivered in Mikrotik-Recv-Limit)
It has no function to change speeds when this limit is reached tho.
ADD: If you have problems with the Mikrotik-xxx-Limit-Gigawords attribute, check your RADIUS "dictionary.mikrotik". Insure it has these entries. My early version of FreeRADIUS does not have these lines.
ATTRIBUTE Mikrotik-Recv-Limit-Gigawords 14 integer
ATTRIBUTE Mikrotik-Xmit-Limit-Gigawords 15 integer