They are pond/bladder snails, usually they come with plants as it was your case. You can give plants bleach/potassium permanganate dips to avoid this in the future, or buy tissue culture.
Do not invest in an assasin snail, they are kind of expensive (at least where I live) and you can easily control the population, or destroy it by just manually getting them out.
I personally just control the population, they can actually be helpful and are part of the ecosystem.
But, if you want to destroy them just crush every snail you see.