more-specifics via IX

I have a few customers' customers, who appear at a local IX. Due to the MLPA-like nature of the IX, I hear their prefixes both at the IX and via my own transit customers. I normally use localpref to prefer customer advertisements over peers' advertisements. There is a customer's customer who is advertising more-specifics at the IX (and using a different source AS, to boot). I can think of a couple ways to prevent hearing these, but thought I should ask for suggestions first. -Bradley