I used to squash the pest snails in my 30g, but eventually I started to think of them as 'part of the family' and felt bad about it - except when they get it into the filter tubing and affect the flow!
I feel sort of irrational; but when you think about it, pond snails are not so different from the snails that many of us keep, look after, and feel sad about if they are sick or die.
At least the pond snails will have a quick death; and they might get eaten by something else if they were out in nature anyway.
And your puffer has to eat.