In this paper, a new rumor spreading SEIR model is proposed with considering both the spreading characteristics of rumors in the real online social network and the epidemic dynamics models with latency. First, the dynamical evolution equations based on the SEIR model are established. Then an efficient immunization strategy--important acquaintance immunization strategy, is given for the inhibition of rumor spreading. Finally, based on the user datasets of Facebook, the SEIR model and its dynamical evolution equations with various immunization strategies including important acquaintance immunization strategy are utilized in the computer simulation for the evolution process of rumor spreading before and after immunization. Simulation results show that SEIR model conforms to propagation characteristics of the real online social network, and the important acquaintance immunization strategy is an optimal scheme to solve the inhibition of rumor spreading in the online social network.