Another option is to go to the next car repair and borrow an angle grinder for 5 minutes to clear the metal surface of your steel plate or pay a mechanic a couple of bucks to do it for you..
Most people have sanding paper at home. All you really need is a bit of 60 or 80 paper that will cost you a few cents and a few minutes of your time and you can flatten the etched letters enough that nobody will be able to decipher it.
Some honorable mentions?
Drill a hole through. It's enough to destroy a few random signs and it's over.
Destroy it using any other tools, like a table grinder, angle grinder, or a dremel tool.
Burn a hole in it using a welding torch.
Easiest and cheapest way? Start grinding it on a concrete pavement and after about 10 minutes of doing so the plate will be unreadable. No tools needed.