i have an se dirt jumper with 4 sealed bearings in the rear hub, can i convert it from QR to solid axle? i dont want the quick release for single speed, i want to ditch the skewer for a solid axle to torque it down good for single speed...  my LBS mechanic said they couldnt do it but with sealed bearings in the hub that seems absurd, any thoughts?
If your LBS mechanic looked at it, and said it wouldn't work, why would any of us second guess a professional on a project which can't even look at? There are many reasons I can think of why doing so is a bad idea, even if it works technically.

The biggest reason I can think of for it not working technically is that an axle for a sealed bearing hub has large, machined areas designed to interface with the inside race of the bearing. A solid axle, unless specifically made for that hub, isn't going to be machined to fit the bearing race.

In addition, most axles for sealed bearing wheels are aluminum, which is fine when using a compression closure like the quick release, but which is a poor, and very likely unsafe, material for a threaded closure system like a single speed axle nut.