Hi, They are removable, but if you are going to be using them without the spikes and on floorboards, I would go to B&Q (one of the big stores) and buy a 300mm square slab of granite slab (they do white speckled granite), you can get them for about £4 each.
Apart from looking good under the stands, they stop the bass and other low frequencies going into the floor space and causing a bass box effect.
If I was you, stick a few big blobs of blue tack under each corner of the base of the stands, and stick it to the slabs, as this will stop them being knocked over.
I have them like this in my bedroom, and as the floor in the longe is concrete, just bang the stand hardish and it gives a good base for the stands to sit on.
Phil