There's no way to inhibit this behavior. Accounts are sort of a "catch-all" for all the various bits and pieces of relational data in the database. An analogy would be the motherboard of a computer, which is responsible for connecting together all the bits and pieces of a computer that make it operate as a computer. That being said, it was once decided that all activities for all records on an account should appear at the account level to give a "master view" of recent activity history on the account. No other object enjoys this privilege, as it is not something you can enable or toggle, but is instead one of the many thousands of small "gotchas" that salesforce.com leverages to specialize their standard objects. This is actually a generally handy feature, since you can get an immediate idea of what has recently gone on with the account. However, as you've observed, sending a mass email can wreak havoc on this list and make it near useless and, of course, deleting the tasks defeats the purpose of the mass email, so it leaves you with a Catch 22 in the usability department. If you really wanted to go about creating a task list free of contact tasks, you'd have to create a Visualforce page to accommodate this.