The latter two were recruited by previous boss Ricardo Moniz last summer and have struggled to hold down a place in the first-team.
Jenner has made 15 appearances for the Magpies this season, although just four of them have been starts, while his teammate has started seven games for the club.
Spencer, meanwhile, joined in February 2014 and scored five goals in 13 games, but suffered a knee ligament injury in July of that year and was out for eight months. He has started just one game this season.
Head of recruitment Guy Branston said: "Jimmy's aware there are clubs chasing his signature so he won't be without a club for too long. He's been a great pro throughout a lengthy spell on the sidelines.
"Civard and Julian have been true professionals throughout and have always tried their best to break into the first team.
"Unfortunately it hasn't worked out for either player in the way we had hoped, and all parties have agreed it is in the best interests for both to move on."
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