The controls are somewhat limited right now. You have two choices
A) Render everything in the scene (default), with HiddenActors removed. Performance cost is usually high because the whole scene is being rendered again.
B) Specify a list of things to render through ShowOnlyActors on the PlanarReflectionComponent. There’s a corresponding blueprint function for things spawned at runtime. Nothing else will be rendered if this list is populated. Any parts of the reflection not touched by the planar reflection will fall back to reflection captures. Typically you would put gameplay components like characters in ShowOnlyActors and try to handle everything else with a BoxReflectionCapture for good performance.
There are also show flags on the planar reflection actor which allow you to turn off potentially expensive features like dynamic shadows.