Clean and reuse fouled plugs

If they still jump a spark (sooty plugs usually do), just run em and flog on the engine to burn them off. The tips are "self cleaning" after all.
You can also clean them with a torch. There are sparkplug media blasters out there too.
If they're too fouled to spark, then media blasting or torch is the better way to go.
If they're iridium or platinum, then a soak with oven cleaner or solvent is the better choice. A nylon bristle brush would help in that case.