  We develop a numerical tool for the fast computation of the temperature and
polarization power spectra generated by domain wall networks, by extending the
publicly available CMBACT code --- that calculates the CMB signatures generated
by active sources --- to also describe domain wall networks. In order to
achieve this, we adapt the Unconnected Segment model for cosmic strings to also
describe domain wall networks, and use it to model the energy-momentum of
domain wall networks throughout their cosmological history. We use this new
tool to compute and study the TT, EE, TE and BB power spectra generated by
standard domain wall networks, and derive a conservative constraint on the
energy scale of the domain wall-forming phase transition of $\upeta
<0.92\,\,{\rm MeV}$ (which is a slight improvement over the original Zel'dovich
bound of $1\,\,{\rm MeV}$).
