Hi, Friends can any one tell me ,

Hi, Friends can any one tell me , is there any way to hard reset a Android based smartwatch like z9 or something just like smartphones hard reset i.e. power button+ volume down button.

In the settings menu - there is an option to factory reset equipment.

This is what is available unless you install custom recovery and wipe and then re-install the rom

Is there any other way to reset .

Not if you do not have TWRP recovery installed - no.
Well - not completely true. You can google reset using ADB interface.