how to remove custom field values

You can delete them through the database. If you have phpMyAdmin available to you for accessing your database, go into the postmeta table (usually wp_postmeta), browse the records, and delete those you no longer want around.

A fast way to hit multiple records at once is to use a SQL query statement like this:

DELETE FROM wp_postmeta WHERE meta_key = 'syndication_source';

Just change the table prefix (wp_) if not using the default. Run a DELETE query on each meta_key value you want to remove.

Warning: Backing up (exporting) your database before doing anything like this is a Good Thing™.

This took forever to find but it was just what I needed to rip out the stuff that the Geo plugin left behind. I never got it to work and it just kept dumping _geo_location: in my pages, with values or without.