Could I restrict only one user can remote monitoring my ADSL Router P3351 / SP3354 / SP3356 / SP3357?
Yes. This series of router allows user setting management via both WAN and LAN. At the Miscellaneous Configuration page, select "Restricted" then check WAN and type in the specified IP address and subnet mask. Enable HTTP password protection. Then only the specified user can access the router to change the setting.