Last Activity Field

Last Activity Field

Is there any way of removing field modified as a criteria that updates last activity.

The whole point of a last activity field is to track that sales people are contacting leads/contacts.

Anytime I do a mass update of leads for any reason its updates the last activity to that date.

I have created a last activity date field that updates using a workflow for tasks/calls/event but its not possible to update the field using a workflow anytime an email is put in using bcc.