We develop a coarse-grained model to investigate the influence of nanoscale particles on the phase separation and the morphology of symmetric AB diblock copolymer films. The microphase separation is modeled by the cell dynamical systems (CDS) equations, while the particle dynamics is described by the Langevin equation. We assume that the particles have a selective interaction with the A block, and we vary the interaction strength, the particle number density, and mobility. The mobile particles destroy the bicontinuous structure that is typical for symmetric diblocks. In the limit of weak interactions between the A blocks and particles, the A's form the continuous phase and the B's form isolated domains. In the case of strong particle/A block interactions, the B phase is continuous and the A blocks self-assemble into distinct islands. Thus, the character of the continuous phase can be tailored by varying the strength of the block-particle interaction.