The following materials are banned for safety reasons, because they give off highly toxic fumes, are a fire risk, or may damage the machine.
PVC/Vinyl - releases Chlorine which will damage the laser
Any other material containing chlorinated or halogen based compounds
Treated Leather - releases Formaldehyde
Any other material containing more than trace levels of Formaldehyde
Polycarbonate
Polystyrene foam
Epoxy/Fibreglass
PETG - releases large amounts of fine dust which can contaminate the optics and is flammable
Coated carbon fibre
Any metals, other than engraving anodised Aluminium
Food - please don't eat anything out of the laser cutter