It's a mess isn't it?! The list of tags in your pic are compiled from each car's ui_car.json file. For the Abarth500, for example, its in
\assettocorsa\content\cars\abarth500\ui\ui_car.json
Depending on what you want to achieve there are tools in CM or you can go straight for text editing the .json themselves.
Beezer has beat me to it for individual cars but for CM if you select all the cars you want to edit in the content tab of CM then go to "Add tag" it also gives you options for removing shared tags. So you can get rid of all "gt3" tags for all cars that have it, for example.
I haven't tried it myself, but if you want to scratch everything and start over with your own tagging system, then I reckon textcrawler would be worth a shot
https://www.digitalvolcano.co.uk/textcrawler.html
If you open up this file with a text editor, the first few lines look like this:
{
"name": "Abarth 500 EsseEsse",
"brand": "Abarth",
"description": "Turning the modern Fiat 500 into an Abarth meant lowering the ride, adding a turbocharger to the 1.4-litre engine (no mean feat under that tiny bonnet), giving it Torque Transfer Control for more aggressive cornering and improving the aerodynamics with side-skirts and spoilers.<br><br>The little Abarth is fast, very agile and plenty of fun.",
"tags": [
"#small sports",
"fwd",
"manual",
"turbo",
"street",
"hot hatchback",
"italy"
],
So you'd just have to get textcrawler to search your whole cars directory for ui_car.json and then do a replace everything between
"tags": [
and
],
with blank.