You could create a calculated field with the measure for utilizing in the top N with the customers specifically identified set to a value of 0.
IF [customer name]='Clark Kent' then 0
elseif [customer name]='Kermit Frog" then 0
use this calculated field for your Top N calculations but use the actual measure (sales in the example) in the view.
See the attached example in Tableau v10. The tab 'without ken and sean' has two of the top customers removed for an example of this.
I hope this helps.
sets-top-n.twbx 1.2 MB