There is no integrated way. But since Hypercard is completely read/write, you would simply choose the button tool, select the link on the zettel you want to remove it from, and delete it.
Backlinks are represented as text in a field, and they are a bit more cumbersome to remove. To remove a backlink you would need to select the field tool, unlock the text of the backlink you want to remove, and erase its characters.