To heat the acorns enough to kill any bugs that might be living inside but also not damage them, you want to use a low-temperature setting for a decently long length of time. Try 175 degrees Fahrenheit for 1-2 hours. You can use the Convection Bake setting to help suck any moisture out of the oven, or use the standard Bake setting and crack the oven door open a bit to let the moisture escape.