  In this paper, we study the four-body decay processes of neutral flavored
mesons, including $\bar K^0$, $D^0$, $\bar B^0$, and $\bar B_s^0$. These
processes, which induced by a hypothetical doubly-charged scalar particle,
violate the lepton number. The quantity $Br\times\left(\frac{s_\Delta
h_{ij}}{m_\Delta^2}\right)^{-2}$ of different channels are calculated, where
$s_\Delta$, $h_{ij}$, and $M_\Delta$ are parameters related to the
doubly-charged scalar. For $\bar K^0\rightarrow h_1^+h_2^+l_1^-l_2^-$,
$D^0\rightarrow h_1^-h_2^-l_1^+l_2^+$, and $\bar B_{d,s}^0\rightarrow
h_1^+h_2^+l_1^-l_2^-$, it is of the order of $10^{-13}\sim 10^{-11}$ ${\rm
GeV^4}$, $10^{-17}\sim 10^{-10}$ ${\rm GeV^4}$, and $10^{-17}\sim 10^{-10}$
${\rm GeV^4}$, respectively. Based on the experimental results for the
$D^0\rightarrow h_1^-h_2^-l_1^+l_2^+$ channels, we also set the upper limit for
$\frac{s_\Delta h_{ij}}{M_\Delta^2}$.
