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Spice can operate (mathematically) at any frequency you care to ask it to operate. What is hard is to produce a meaningful circuit description that makes sense when interconnections become more than a fraction of a wavelength at the analysis frequency.
When wires act like inductors, when wires have capacitance to every other nearby node, when wires act like rf transmission lines, and the models used for semiconductors and integrated circuits only hold for frequencies from dc to a few tens of MHz, it is hard to create a circuit model that reflects reality at 100s or 1000s of MHz.
Spice has no inherent limit to the simulation frequency.
It's limited only by the circuit models used in the simulation.
You can have transistor models, for example, that go up to the GHz region and can thus be simulated at that frequency.
The real circuit problems come with all the parasitic inductance and capacitance of a physical circuit as Mike noted.
If you can make a good estimate of those and put the into the simulation, then you can more realistically simulate high frequency circuits.