I'm having issues with this too. I am playing as a Hive Mind and I am unable to purge off 75 robot pops I've captured. Hive mind has no options for robotic workers policy, and there are no options to purge under species. I also think it is interesting that they are considered livestock and they are providing food resources. I don't expect this is intended behavior.
I also have a lot of different pops, invaded a zenophile. I wish I could just have a mass project to convert all non-main races into a single nerve stapled worker drone race, it is very tedious making the same worker drone template for all the different races and trying to manage them.