Sanding. An orbital or dual-action sander will take off anything in its way, including decals.
Scraping. The combination of a heat gun, razor and adhesive remover (and a lot of tedious work) will eventually get the decal off.
3M Stripe Off Wheel: I stumbled upon this gadget while browsing Amazon one day. It's essentially a giant rotary eraser wheel that promises to take off pinstriping, vinyl, tape, adhesive residue...and decals. Works great on metal without damage to the paint. Unfortunately, it can't be used on plastic — it will gouge it. I ordered one up for $25 and gave it a try. Here's the Amazon link:
https://www.amazon.com/gp/product/B00063VT0G/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1