  We study the thermal properties of the classical antiferromagnetic Heisenberg
model with both nearest ($J_1$) and next-nearest ($J_2$) exchange couplings on
the square lattice by extensive Monte Carlo simulations. We show that, for
$J_2/J_1 > 1/2 $, thermal fluctuations give rise to an effective $Z_2$ symmetry
leading to a {\it finite-temperature} phase transition. We provide strong
numerical evidence that this transition is in the 2D Ising universality class,
and that $T_c\to 0$ with an infinite slope when $J_2/J_1\to 1/2$.
