Jack Voldemort is the nickname OLR gives to a certain anti-video game activist and attorney.
The usage of nicknames to refer to him stems from the idea that he is seen as an attention whore, drawing controversy to various video games with the intent to gain attention. As such, OLR and its listeners try to avoid addressing him by his real name to avoid giving him any further publicity.
Previously, for this purpose, OLR nicked Jack as "He-who-shall-not-be-named," after Lord Voldemort from the Harry Potter series, the main villain of the series and a wizard so feared that even the mention of his name strikes up shivers among others.
One day, in #orangelounge, someone, noting that Jack was being refered to as He-who-shall-not-be-named, equated him with Lord Voldemort and came up with the nickname "Jack Voldemort." The name stuck.
Other nicknames for Jack include "Jackie-poo," "Tack Jhompson," "Crack Potson," among other things, though he is usually referred to as Jack Voldemort or simply Voldemort.