How to stop trackbacks?

At least a dozen times a day I get notifications to check incoming comments like:

“A new trackback on the post #72 “From this morning’s iChat with Hao-chan” is waiting for your approval”

They are always 100% spam, selling drugs or porn or some-such.

I go in and mark them all as spam, but is there any way of stopping them from coming in to begin with?

In Dashboard > Discussion I unchecked the box “Allow link notifications from other blogs (pingbacks and trackbacks.) ” the other day, but that seems to have no effect on these coming in.

Thanks,

doug