Drip some PB-blaster down the spoke into the nipples and let sit overnight. If there's still enough spoke-nipple sticking out to get a set of needle-nose vice-grips on, try that.

Yeah, alloy nipples do corrode faster than brass due to the higher difference between steel and aluminium on the anodic scale. Best to use spoke-prep, Loctite or boiled linseed-oil on the threads to provide an insulating later between the two materials.