The information in this post is based on Dynamics AX 2012 R3. It may or may not be valid for other versions.
If you need to hide a data contract field from an automatically generated SysOperation dialog, you simply need to apply the SysOperationControlVisibilityAttribute attribute to the corresponding parm method. For example:
[DataMemberAttribute, SysOperationControlVisibilityAttribute(false)] public ItemId parmItemId(ItemId _itemId = itemId) { ; itemId = _itemId; return itemId; }