B22 is right, although beardie poo is pretty nasty (its from all the roughage!) it shouldnt be so bad that its causing you a serious issue.
First step would be to get a fecal done, if parasites are causing the smelly poo then no substrate is going to help.
Sand is not an idea because it is going to harbor that bacteria and is very tough to keep clean, i would suggest a couple of things...
1. Shelf liner (i know you said you tried it, but i dont know how you were cleaning it) if you wipe up the feces with a wet paper towel and toss it away then it shouldnt take long for the smell to disperse.
2. Bath training them. There are many beardies who mainly defecate in the bath, including mine. You should be soaking your beardie at least weekly, so just up that to 2-3times a week and extend the baths for as long as your beardies will stay in. Most of them naturally want to defecate in the water, and if they are provided with enough oppurtunities to do so, they will eliminate almost always in the tub!!
Good luck!
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0.1 Bearded dragon (Hannabil)
0.1 mexican Black kingsnake (Morticia)
2.1 Leopard Gecko's (Pogo,Jeffrey Nothing, Louise)
0.0.1 Tokay Gecko
1.0 Blue Tongue Skink
0.0.1 Reverse Okeetee Corn (Autumn)
1.0 Blood Red Corn (Virus)
0.1 Bullmastiff (Asha)
4.1 Cats (Poe, Tucker, Abhid, Felicity, Emmy)