if memory serve me correct (zoology in high school) hydra's reproduce asexually, meaning they split/divide like bta's, yes they are bad for the tank as you dont have the critter that actually eats them! in the home aquarium they most not be cut or touched due to stinging cells( related to jellyfish ) they may not break the skin but if you had an open wound it might is a large enough stationary colony stung you. Remember the next time your at the beach these guys are everywhere,both stationary and free swimming( thats when there called medusa's), but nearly most( 99.9% ) are not harmful to humans. but to get to the point your going to have to kill the host!
p.s. putting a critter in the freezer is more humane then smashing it, it well at least go into hibernation and die in its sleep with lil to no pain.
if you smash it, it's like you being hit by a big truck you wont die that very second but youll feel alot of pain before you do!
one more thing to remember pet stores freeze feeder mice/rats as to just smashing them! just food for thought!