A Paradox Shield was a device created by the War Doctor to prevent Marinus from being attacked by the Daleks during the Last Great Time War. (COMIC: Four Doctors)
The woman who installed the Parnaeum in the Stagnant Pond placed a paradox shield so that it could not be removed by anybody who existed. However, Gideon Lyme was able to do so given that he was supposed to have died. (AUDIO: Parasite)
A Paradox Shield protected Lara Zannis, a human agent of the Daleks who invaded Earth and exterminated humanity in 1961. Once the shield was broken, Lara was banished to this alternate timeline when the War Doctor restored the course of history. (AUDIO: The Shadow Vortex)