It is heart breaking to see damage to the bark of trees caused by strimmers. After all the hard work that has gone into selecting the right tree, paying for the tree, planting the tree and then caring for it to ensure it establishes a moments error by a strimmer operator can cause irreparable damage, to a tree.
If a very bad case or repeated damage, this can kill the tree.
Training operators of strimmers can be on great benefit.
Mulching 3" deep as wide as you can, with the immediate area around the trunk kept clear, can keep the area weed free, negating the need to strim. Mulching also has massive other benefits for the tree.
Protecting the lower trunk with a strimmer guard will prevent damage. These are available from suppliers, but you can also use a length of plastic drain pipe but vertically split to allow for future growth. We can put you in touch with suppliers.
Weedkiller can be carefully applied but we would rather not recommend this, it needs to be applied as per manufacturers recommendations and cannot be sprayed on the trunk (as the trunk will absorb it).
You could weed by hand (if a reasonable task)