The reject list only works for Calls and not texts!!!
Well text there number you need to find there number first and tell them it should work it worked with my 02
I rang o2 customer services for help and they were useless, told me I couldn't do it so I just tried Soweto's suggestion so we'll see if it works, thanks a lot, much appreciated.
On Call log press Reject List - phone shows you all numbers from txt, calls made and received, then you choose the one you want to block- then hit ADD you're done.