I don't want my fish to breed!

[Baker]

Well just because your fish likely choked to death on a snail doesn't make snails dangerous. And the snails themselves don't possess any natural biological processes to make them toxic or anything like that (pond snails at least)
The infestation is caused by excess food in the tank, you can easily control the populations by cutting back on feeding. Fish don't need as much as you think to survive.
And why would a store need to tell you their planted tanks have snails? It's practically assumed that they do as they are extremely beneficial

 

[snork6]

" you need live plants as well as the moss balls to make your tank work better."

a common known fact in the hobby is that plants come with snails, if you don't want them you must take the necessary actions to remove them before placing the plant in the tank. (I'm not saying its your fault for not knowing that, I just wanted to make a statement for people reading this to inform themselves)

Who ever gave you advice was poorly educated about live aquarium plants, or just didn't finish telling you all the necessary information.