Unfortunately, there is no default means or some button to remove images from necessary products in bulk.
This is definitely a bad idea to do it directly in DB, as deleting/clearing the tables may affect consistency of your data and you will have to remove images from media/catalog as well.
So it is better to use a script (if you have some tech skills and experience) or extension (if you don't). In second case I can recommend MagicOne Store Manager I used. It has option to remove images for selected products before import or clear old image gallery right when you upload new pics. Here is tutorial on this.
It is not free, but I guess you need this task to be done one-time, so free version should be enough for you.
