From the FAQ
Q: "What about satellites? How do they orbit the Earth?"
A: Since sustained spaceflight is not possible, satellites can't orbit the Earth. The signals we supposedly receive from them are either broadcast from towers or any number of possible pseudolites. However, temporary space-flight is possible. The gravitational pull of the stars and other celestial bodies balances out with the UA and thus allows spacecraft to float freely (Diagram).
Thus, if orbiting the Earth is impossible, it is impossible to leave the Earth's orbit either.
But there is another conspiracy theory in which some people do not believe in the Moon landings. According to NASA, Apollo 11, Apollo 14 and Apollo 15 have left back corner-cube retroreflector on the Moon. The McDonald Observatory and the OCA Laser-Lune telemetry station affiliate with the Cote d'Azur Observatory shoot laser beam on Apollo sites everyday and the laser is reflecting back on Earth (that's how we know the distance between the Earth and Moon). Moon Hoax Believers claim that it is probes that left back these retroreflectors on the Moon (like the unmanned Soviet Lunokhod 1 and Lunokhod 2 rovers).
But no matter if it's Apollo missions or probes that left back retroreflector on the Moon, the fact that they are on the Moon today and still using by observatories in the world today is the proove that it is possible to leave the Earth's orbit. That doesn't proove the Earth is round, but it's a proof that FE theory is wrong again.