this is dictated by the output style you are using and when you are using the “pages” field in the edit citation dialog. A quick fix might be to put exactly what punctuation and page numbers you want in the suffix field, instead. The long term fix is to edit carefully the output style you are using (using link adjacent and separator characters so the punctuation is only kept when the page field has content), save to a new output style name and change to that output style in your manuscript.
What output style are you using?