"Stop up" is a phrasal verb that means to block or obstruct something. It may also imply closing or sealing an opening. However, when writing, it's essential to have a vast vocabulary to avoid using repetitive words. Therefore, here are some synonyms for "stop up" - block, seal, close, clog, obstruct, jam, impede, plug, stuff, fill, dam, halt, barricade, slow down, stem, restrain. These synonyms can enhance and add depth to your writing, making it more captivating and engaging to read. So, the next time you want to stop up a hole, consider using any of these synonyms to make your writing more diverse.