Prevent to create opportunity when qualifying Lead in CRM 2013

Use the following code in your plugin and register in the PostValidation on the QualifyLead message for the Lead entity.

context.InputParameters["CreateOpportunity"] = false; // set to true by default
//context.InputParameters["CreateAccount"] = false; // set to true by default
//context.InputParameters["CreateContact"] = false; // set to true by default