Filter in/out?

Using the filter command in the cdr_adaptive_odbc.conf, is it possible to filter things out, instead of filtering them in?
I have my extensions.conf set up to feed a custom CDR column that will be registered as blank if the callerid isn’t a number (ex:blocked, unknown). I’m looking to filter out all calls that have a blank for that custom cdr. Is there a way to filter all call out of the cdr based on the empty custom cdr?

If its not possible to filter things out, I tried using variables (_XXXXXXXXXXX) like I can in the extensions.conf but that didn’t work.
If I could use variables like _XXXXXXXXXXX I could get it to write the CDR.

Is this possible or should I start looking for a work-around?

Thanks!