Constrain in positive Z plane only?

I've not seen it mentioned anywhere so I must ask. When in 3D modelling, is it possible to set AutoCAD to constrain drawing and movement in only the positive Z axis? I'd like to draw obviously in the XY plane but also only in positive Z (above ground?). I don't think i'll ever need to draw anything in the negative Z plane, which by my understanding would be actually underground (correct??). If it is at all possible to constrain in the positive Z plane, would there be a way to temporarily over ride this for the times I may need to Press Pull a hole out of a solid?

I have this strange knack of drawing in Top view in a left viewport (model space) then extruding in the right SW viewport then sometimes when I try ti move objects the end up underground (negative Z plane).