So sorry to hear this sad news :'( i've been reading this thread over the last few days so it's upsetting to hear that this happened.
Please let us know how you get on with the RSPCA. If they can't do anything, myself, and i'm sure a lot of other people on this forum will be more than happy to write a letter to the garden centre. I think it may get the message across that what they are doing is wrong.
Christine xx