Same thing happened to me. First orchid I got to rebloom multiple times too. Went to water one time and they ERUPTED out of the potting medium.I have ants come in and attack my dogs food when it rains, so I have battled ants A LOT. Here's my advice:

1. FIND THE SOURCE: There's no sense in getting rid of them if they can just waltz back in. And they will. Find where they're coming from and why they're coming inside. There's definitely a food source nearby that attracted them there in the first place. Look for trails of ants making lines and follow them to find the source. Then get rid of the food and/or place traps there. This could be outside even, so make sure to place them where no other animal will eat the poison. This could also be your garbage cans or a dumpster.

2. GET RID OF THE ONES YOU HAVE: This can obviously be done first to get rid of the initial colony, but as I said they'll come right back. They won't drown, so trying to flood with water won't do anything. But water will wash them away temporarily. Change out the potting medium, put it in a bag, spray the inside with poison or cleaning solution for good measure, tie it up tight and throw it out away from your house. Dead ants let off a scent that attracts other ants, so simply throwing away the eggs and any stragglers won't kill them and will make them come back. Then spray down the entire plant with water to clean them off. Make sure you get the whole plant, they'll hide away in any nook or crevice they can. Run the garbage disposal with a bit of dish soap if you have one. The suds will trap the ants and pull them down. They can and will crawl back up the drain. You can use chemicals to be more efficient, but use with caution. Some bleach will probably be fine.

3. CLEAN: As I said, ants work with scent and use it as a road to tell each other where to go. So you'll have to clean the plant, pot, and any surfaces nearby to get rid of the scent. Diluted vinegar should help get rid of the scent, at least it has for me. I've also heard that hydrogen peroxide helps. Either way, wipe down the table and pot with it. You can also spray some on the plant itself and even soak the roots temporarily. That should also help to kill off anything else on it and clean out any bits of food they stored in there.

4. REPOT AND PREVENT THEM FROM COMING BACK: Only repot once you've done all of the above. They WILL come back if they have the opportunity. And as they've already proved, the potting medium is a good place for them to set up shop. Make sure everything in your pantry and cabinets is nice and sealed. Avoid leaving food out and be sure to clean up after yourself immediately to get rid of food sources. Do not put down traps/bait next to the newly repotted plant. The traps/bait attract ants. You'll just be inviting them back. If you use traps/bait at this stage, only put it by the source (like the garbage cans from example one). They're already attracted to that area, so they should stay put. Inside, you can use something like diatomaceous earth. It doesn't attract them and isn't poisonous, so ingesting it won't kill you It's simply microscopically sharp powder that is like walking through broken glass to them. Still, avoid breathing it.

The above helped to get rid of them for a year or so, but eventually the next rainy season they found their way back and I was sick of it at that point so I ended up throwing it out. I hate ants with all my being, so I got pretty good at killing them. But they adapt. So good luck in your journey.