This is due to the way the caustics calculations are integrated with global illumination.
Unfortunately, there is no reliable way to distinguish weak caustics from the overall GI in the scene.
While there are ways to heuristically attempt to discern caustics from everything else, this is not at all reliable and led to the engine incorrectly determining which is which in our internal tests, and was hence abandoned being an unfeasible mechanism.
The only way that caustics casting could be prevented from a specific light source would be to disable the caustics solver for the specific light source and, although already implemented, this still will not prevent path tracing from calculating some forms of caustics, hence making the ability to completely disable all types and kinds of caustics technically impossible without simultaneously disabling GI for the whole render.
Examples
1. Caustics solver enabled in the Performance tab. "Generates caustics" enabled in the light object.
Both reflective and refractive caustics are generated as expected:
Nonphysical properties rollout of the Corona Light:
2. Caustics solver enabled in the Performance tab. "Generates caustics" disabled in the light object.
Note that even though the caustics solver is disabled in render settings, both reflective and refractive caustics are still visible. This is because they are a part of the GI solution, and in this case are calculated using the standard path tracing solver (which makes them noisier and slower to render).
Nonphysical properties rollout of the Corona Light:
3. Caustics solver enabled in the Performance tab. "Visible in reflections" disabled in the light object.
The caustics solver is enable in render settings in this example, however only refractive caustics are visible, because the light is ignored in reflections. Note that in this case the light is not visible in reflections on the metallic cube object.
Nonphysical properties rollout of the Corona Light:
4. Caustics solver enabled in the Performance tab. "Visible in refractions" disabled in the light object.
The caustics solver is enabled in render settings in this example, however only reflective caustics are visible, because the light is ignored in refractions. Note that in this case the light is not visible in the refractions of the glass object.
Nonphysical properties rollout of the Corona Light:
5. Caustics solver enabled in the Performance tab. Both "Visible in reflections" and "Visible in refractions" disabled in the light object.
In this case, no caustics are visible at all, and the light is also invisible in reflections and refractions.
Nonphysical properties rollout of the Corona Light:
6. Caustics solver disabled in the Performance tab. All checkboxes in the light object enabled.
In this case we are using the standard path tracing solver to calculate any caustics. Note that the result is identical to the example 2 where the caustics solver is enabled in the Performance tab, but is not actually used in the rendering.
Render Setup > Performance tab:
Nonphysical properties rollout of the Corona Light:
7. Caustics solver disabled in the Performance tab. Both "Visible in reflections" and "Visible in refractions" disabled in the light object.
Similar to example 5, no caustics are visible at all, and the light is also invisible in reflections and refractions.
Render Setup > Performance tab:
Nonphysical properties rollout of the Corona Light:
8. Using the RaySwitch map (or material)
To disable reflective or refractive caustics for any material, you can use the RaySwitch map or material with a black color/material set in its GI slot. This way, no light will be ever reflected or refracted by this material, regardless of using the caustics solver or pure path tracing and regardless of the Corona Light's settings.
In this case, a RaySwitch map was used to disable reflective caustics for the metallic cube object (plugged into its reflection color slot), and the same RaySwitch map was used to disable both reflective and refractive caustics for the glass object (plugged into its reflection color and refraction color slots).
Material setup: