No. Your sponsor cannot cancel your visa but they can terminate your employment. If you do stop working for your current sponsor, you have 28 days to find a new sponsor and apply for a new 457 visa. You must phone DIAC on 131 881 or approach a DIAC business centre for advice within this 28 day period to ensure your visa remains valid. You may also have the option of applying for another visa subclass. If you do not apply for another visa, you must leave Australia before the end of the 28 days, unless you have received approval from DIAC for an extension.
If your employment has been terminated by your sponsor, DIAC must be notified. Your sponsor may be required to continue paying you for a period of 28 days.