You can give them pepto bismol, imodium or kaopectate--but you have to know the measurements for it--unless it's a Collie. I don't know why, but collies and some other breeds can't have it.
I'd quit the regular food for 24 hours, give regular-white-rice-water (just boil regular white rice in water until the water is creamy white and strain) and give small amounts of pumpkin. Save the rice and mix with cottage cheese after the 24 hrs are up. Add boiled chicken to the rice and cheese the next day. The stomach needs time to settle.
When dogs won't eat the pumpkin cure-all, I treat them like kids and give bananas. Bananas are a cure-all for kids with constipation OR diarrhea. I wouldn't give psyllium because it blocks things up instead of letting whatever's bad go away. Many of these things need to run their course.
This is a chart that I have MY vet's permission to use for MY dogs. They don't have the weird genes that are dangerous like collies though.
Note: NONE of these things worked either time when they got Giardia, but my signal for that now is a memorable, strange smell.