HI,
I think you should have at least a date column in your table, with this date you can show only the latest entries and for instance delete anything that is older than a certain period.
You could try something like this:
"SELECT columns FROM table ORDER BY date_column DESC LIMIT 10"
The most recent entry (comment) will show up first in the result set and you limited the result set to a maximum of 10 rows.
Good luck,
GreetZ,
Joris