Flutter can and will occur anytime that an external input occurs at the natural frequency of any structure. Now all structures will resonate at some frequency, even an iron bar or a concrete bridge. If that structure is excited by a force at that frequency of resonance, flutter can result. The only thing that will not flutter is something having infinite rigidity and no mass, a physical impossibility. You eliminate flutter in your model by changing the natural frequency of a structure on one that is not ever excited by changing stiffness and/or mass which shifts the frequency to one not excited by flight conditions. This can be done by an almost infinite number of ways, the simplest being to either stiffen the structure or change it's mass or its plan-form. Closing the gap with tape increases the stiffness, adding counterweights changes it's mass, changing shape (removing or adding area or redistribution of area) are all possible solutions. If you know the source of the stimulus, of course removing that will also solve the problem (like changing or reducing speed which changes turbulent patterns).