You can remove individual formats from the galleries, as Dihao Guo suggested, or you can reset all the numbering and bullet galleries to the default by exiting Word and deleting the ListGal.dat file in your user profile path.
However, a long-term solution for bullets and numbering formats would be to define styles for them. See https://shaunakelly.com/word/bullets/controlbullets20072010.html and https://shaunakelly.com/word/numbering/numbering20072010.html.