Bearing in mind that I’m using Sketchup Free, is there any way of preventing the reflections that you can see at the corners of this logo I made (where the blue turns to a lighter shade) ?
You’re referring to the shading due to the 3D-ness? No. In the SketchUp Free there isn’t an option to remove that. In the desktop versions you can, though.
Typically when I have that bleed through I edit the back face color to reduce or eliminate it but that is also not something you have as an option with SU Free.