It is inevitable that you get pest snails along with any plant. I've purchased plants from 'reputable' companies and I get snails. I treat every plant before it goes into the tank with Reverse Respiration.
www.reverserespiration.com
It places plants in a high pressure, pure-CO2 solution that asphyxiates the snails, worms, eggs, aerobic bacteria and insect larva but actually enriches the plants with an intense CO2 environment. This is done for 12 hours, in the dark.
The plants are then placed in plain water (an O2 environment) for at least 30 minutes which suddenly changes the pressure which kills off any remaining eggs or anaerobic bacteria (if any). This last step also helps destroy any remaining algae.
You’ll need as much seltzer as is required to fully submerge the plants you are cleaning. It makes the plants quite buoyant such that you’ll need to weigh them down to keep them submerged. I use forks to hold them down.
1-Rinse the plants. Fully submerge the plants in freshly opened seltzer water.
2-Cover the top with a towel or other loose-fitting lid (should not be airtight) and place in the dark for 12 hours.
3-After 12 hours, remove the plants and soak in plain water, preferably aerated for 30 minutes or more. Usually, tank water is sufficiently oxygenated such that you may skip this step.
4-Rinse to remove remnants of pests or dead algae.
5-Place in the tank.
I had a severe case of black beard algae and RR killed it all and has never returned.