You can, but there's not a whole lot of good things to spend it on. Marginal increases at best (imp cleave, anger management, imp execute, etc). If you're at 18 expertise, it's better to just wear less expertise if you can; at the very least, keep your eyes open for non-expertise pieces to replace some of your slots.
Perhaps not 0, as u can still benefit from it, until the hard cap value, as u can take the points on weapon mastery and spend them elsewhere. But it should have a lower SEP value, or 0 if you don't care about the talent points.