Sap is not spammable anymore , boohoo. Well, here is your solution!

Simply make this macro:

#showtooltip Sap
/targetenemy
/cast Sap


Spam this in arena, and it will sap anything stealthing around you (it targets it and saps it).
This also works in BGs, but because of the great many people around you, the /targetenemy may screw things up.

Happy Sapping.