How do you reflavor nuts?
If you want them to taste like cashews, there should be no need to "replace flavor" - you can attempt washing and then drying in the oven - results may vary depending on how the "spicy" was applied. Probably they will remain somewhat spicy, as those flavors tend to be fat-soluble, so they will get into the nut but not come out with a wash (and I can't really see "washing in oil" as an effective or economical technique.) 

Giving them to someone that likes spicy things would be another option, and more reliable. Then buy some that are not spicy for you.