How to get Garlic & other strong Spices' smell out of my Zojirushi Flask/ Thermos acquired via a herbal Immunity decoction?

Baking soda and vinegar. Add as much vinegar as you want and then add a teaspoon of baking soda. Once it's finished bubbling you can add another teaspoon and another. If there is still an odor repeat. It's amazing how well this works. I clean gas engine carburetors with this solution.

I the smell is stuck in there after many months, you have to decide if it's usable as is, because if not it's time to try riskier methods. In no particular order, I would try:

• Soaking the offending part in vinegar for a few days
• Soaking in a strong solution of bicarbonate of soda for a few days
• Soaking in hot water for a few hours: put the seal inside the flask, fill with boiling water, and put the lid on (keep upright), or put in a bowl of boiling water, with a lid, and place in an oven at 60--100°C. A few drops of detergent would be a good idea.
• Soaking in diluted bleach for a few hours.
• Covering in cooking oil for a few hours. Some of the flavour compounds are likely to be more soluble in oil than in water.

In between all steps, wash in hot soapy water, or even better a dishwasher, even if it says not to.

Note that may of these are specifically against the manufacturer's guidelines, but are unlikely to do real damage (they may cause premature aging or discoloration). These slightly risky ideas are mean for something that's otherwise going to go to waste.

If it's true silicon rubber, I've had some success baking my Instant Pot gaskets at 350°F/180°C for half an hour. It doesn't remove the smell completely, but certainly reduces it by quite a bit. It's in the nature of this material to absorb odors, and there's not much you can do about it. I have separate gaskets for savoury meals and non-spicy things like cheesecake or oatmeal for this reason.

For something like a water bottle, I'd just buy a new one at this point, and make sure it's single use. E.g. only water, only coffee, or only strange concoctions.

1. Wash out with warm soapy water. A toothbrush could help in the scrubbing. While still wet, add baking soda into the flask and scrub.
2. Pour an inch or two of white vinegar into a clean carafe, then fill most of the way with hot tap water.
3. Not tried this one, but seems a few people have had good results. 4 Alka-Seltzer tablets with water.
4. Adding freshly ground coffee, which will help to mask the smell if it is still there.

Edit: Just read through the manufacturer's instructions, and have updated my advice as a result.