That seriously sucks! I've had stuff poured in tanks, but it was by a little kid that thought she was helping clean the tanks.
I'd probably get new substrate, definitely get rid of any wood that was in there, there's no telling how much chemical the wood soaked up, nor for how long it would release it. Vinegar might work on the seashell, but be ready for the chemical reaction between acid(the vinegar) and base(the shells) It will probably foam like mad, but I'm thinking that could be a good thing in this case.
I would get rid of any pads or filter floss, they could hold onto chemical and it's pretty certain it already killed your beneficial bacteria, in many ways they're more fragile than fish.
White vinegar is a good biological cleaning agent and it's fairly easy to tell if further rinsing is needed due to the stink.(Plus it's really cheap if you get the store brand!)
A bleach solution one part in ten of water is usable in a situation like this, but rinse like crazy and let dry thoroughly. The chlorine will go to gas form and evaporate. The other option to drying is just adding some extra dechlorinator, but you still have to rinse like crazy. I'd probably do both because I'm slightly paranoid.