Hi Tomas, most tags fire a purchase event based on the _corder variable being set. This is a "special" data layer variable which is created by the Ecommerce Extension to hold the order id. If the order id variable is populated, then the tag will fire a purchase event. If it's not populated, then it will not.
You can disable ecommerce tracking for a specific tag by creating a Set Data Values extension and scoping it only to the tag you want to disable.
You want to set the variable "_corder" using a "Text" value and leave the value field on the right empty.
Because the extension is scoped to a specific tag, then it has a copy of the data object, so the order id is deleted for the specific tag, but not for all your other tags. This should work for any tag that is ecommerce-enabled, and hopefully you won't need to edit the tag template.