Poor Jodie. What a difficult condition for you to have to manage too.
I've never had bitches, so I don't know if there is anything left after spaying that causes hormone production. But I know that humping has many causes, including simple playfulness and excitement, anxiety about what other dogs may be doing and stress. Some would also say dominance, but I'm not so sure about that.
If the humping is after she's been fed, is that a time when she likes to play or is especially active? My two often have a funny 5 minutes after eating and it often ends with William (who is castrated) humping Louis (who is entire)
. It usually happens if Louis has been running about the house and getting silly and barking. I think William finds that a bit worrying and the humping is his way of trying to calm Louis down. That's not as daft as it sounds, because dogs getting anxious that a situation is getting out of hand will try various behaviours to calm things down.
I'm just wondering if that's why Jodie is humping? If so, or even if all you can identify is that it happens after she's been fed, could you give her a quiet half hour or so separated from the other dogs after she's had a meal? It might help simply to curb the gas build up just because she's quiet, or it might help as it will stop the humping.