I am the only who create a tasks for my workers in Trello. How can I prohibit changing of the task by my workers? They must only comment and close the tasks.
This is not possible. Anyone who can close cards can also change them/add new ones.
It will not really cover your requirements, but Trello Business Class provides some additional authorization rights. For example, people can be invited to private boards as observers and they’ll have read-only access. But read-only access does not allow commenting or closing.