since the guide linked by Peter only states to show examples on how to remove AdGroup Bid Modifiers but fails to actually provide such an example, here how it should work (using the Python library):
ad_group_bm_service = googleads_client.get_service('AdGroupBidModifierService')
operations = []
ad_group_bid_modifier_operation = googleads_client.get_type('AdGroupBidModifierOperation')
resource_name = f"customers/{customer_id}/adGroupBidModifiers/{ad_group_id}~{criterion_id}"
ad_group_bid_modifier_operation.remove = resource_name
operations.append(ad_group_bid_modifier_operation)
ad_group_bm_response = (
ad_group_bm_service.mutate_ad_group_bid_modifiers(
customer_id=str(customer_id), operations=operations)
)