Level structures on abelian varieties and Vojta's conjecture

Assuming Vojta's conjecture, and building on recent work of the authors, we prove that, for a fixed number field $K$ and positive integer $g$, there is an integer $m_0$ such that for any $m > m_0$ there is no principally polarized abelian variety $A/K$ of dimension $g$ with full level-$m$ structure. To this end, we develop a version of Vojta's conjecture for Deligne-Mumford stacks, which we deduce from Vojta's conjecture for schemes.