The only way to prevent ETC hours from automatically populating as soon as the resource is assigned to a task is to assign the resource to the task when that resource's allocation on the team tab is still 0.
If the allocation on the team tab is greater than 0 when the resource is initially assigned to a task, ETC hours will be added to the assignment that gets created. The ETC amount that gets added will be calculated based on the allocation amounts. Note that updating the allocation on the team tab will not automatically update the ETC on the assignments if these team allocation updates are made AFTER the resource is already assigned to the task (with the exception of an effort task, which is a task having an ID of ~rmw).
KB000053503: What is the Effort Task in Clarity PPM and how is ETC calculated on it?