Something like

if ($1 == 99.0) return null;
else return $1;

The above would replace the 99.0 with null. Technically still "bad data" and I admit I did not test using null (I tend to set "0" or some descriptive error, depending on what I'm collecting).

To remove data already collected, you either have to either clear the collected data (which will wipe your good data along with it) or go into your database, locate the bad values and remove them.

Hope this helps.