There is no formal way to end a series on this site. On another site, you have to mark the final chapter as "Story Is Complete." You always have the option of adding a new chapter and starting it up again.
Sometimes it's just easier to add sequels with different titles and link each one with the one before it. Or one can link to all the previous stories. I had a stand-alone story on yet another site that grew, in effect, to a series of about eight chapters that way. I even had a plausible ending for it.
On that third site, I once waited six months before I came up with Chapter Two. It eventually ended at ten chapters. When I wanted to later add an additional event in the middle, I made it a stand-alone story because the numbering system could not be changed. I told the readers where it fitted it and they seemed to be satisfied.