A brute force way could be to create a new table, move the data you wish to keep into it, drop the old table and then rename the new table and recreate links, indices, etc. The suggestions from other people are probably worth trying before this though as your system might be off the air for a few seconds whilst the data shuffle occurs. To minimise this you could

1. Create the new table

2. Drop FKs on the old one

3. Rename the old table and the new table (so the new table takes on its permanent name) - new inserts can go in to the new/empty table

4. Move the data

5. Drop the old table