When you add RelationshipTypes to your source ItemType you may open each created RelationshipType (for example use "View RelationshipTypes" in context menu) and set

On New Related Option

Pick Only Create Only Pick & Create

If this is often task then you may prefer edit ItemType with name RelationshipType and unhide property related_option. To do that uncheck Hidden2. After that related_option will always present in relationships grid when you create or edit ItemTypes.