SugarCRM CE 5.0.0C

What is the best way to allow someone to opt-out of e-mail or unsubscribe from an email newsletter using the web-to-lead form (and, perhaps without it). some of the issues are that web-to-lead creates a new lead record on each submit regardless of a previous record. One could have a lead record with and without opt-out checked. The intention may be for a contact record to be changed, rather than a lead record, but the web-to-lead form only communicates with the Leads table. The customer may have subscribed to an email newsletter and now wish to unsubscribe. Other than manually, is there an accepted way to handle this case?

I'm eager to hear how others handle these issues. Thanks.

Mark