In general, your error relates to having a dependency. If you want to erase everything, you can do this one of two ways.
1. Remember your relationships by writing them on paper. Remove them. Do the delete on all participating tables. Manually re-establish your relationships.
2. Draw a dependency diagram. Where you have 1/1 or 1/many relations, the one on the LEFT of the relationship diagram is generally the "parent" and the one on the RIGHT is usually the "child." Delete the contents of all child tables first. Then you can delete the contents of the parent tables. If you have multi-layer dependencies (grandchild tables?) then they go before the child or parent tables. In other words, think of the main table as the main trunk of the tree. Trim the tree from the branches towards the trunk.