Take it off the ISP cable and reset the factory defaults.... usually a pinhole switch. Go in and make a new admin account with a new password. Add it back to the ISP cable. Also change your WIFI admin account and password as mentioned. Some routers will let you authorize users by MAC address also if you're feeling really paranoid. Change your broadcast to ID to something more creative like FBI/ATF roaming surveillance or K9-Unit 54

or just turn off your broadcast all together.