Shipping selection

-- url of the page with the problem -- : dailybread.co.uk/
-- HikaShop version -- : 5.0.3
-- Joomla version -- : 3.10.12

We have several shipping methods which all work fine.
However one is 'group orders' which customers if they belong to a buying group (we have several) get free carriage.
This has worked fine but we are seeing more and more people choosing this as it gives free shipping, when they should choose another method and be charged shipping.

What would be the best option to prevent this happening and how.

We could collect all those customers put them in separate user groups as needed and they would then on checkout see the group delivery option only, (hence removing from the normal selection).

I am not sure how to do this. I can't see in shipping method see a option that only displays if member of a group.

I an guessing I am answering my own question here, but would you use access control in the shipping method?

Or is there another easier option?

Thanks

John