This article demonstrates how to delete old data from a MySQL table and recover the disk space from the removed data. This works with MYISAM databases.
USE mydatabase; DELETE FROM mytable WHERE time < NOW() - INTERVAL 1 WEEK;
The above will remove anything older than 1 week. We're relying on there being a column with the date-time format.
OPTIMIZE TABLE mytable;
The above will reclaim the disk space.