I've seen countless posts of people getting fountain farmed until they abandon recently. If you have links to a game where it happened, post them below and I'll add them to the post. Icefrog has shown he doesn't want to instakill fountain divers, and I think if you get someone to 10%, they run to fountain, and you can quickly kill them, I agree, you should be able to fountain dive to get them. However, constantly sitting in fountain is toxic, farming people until they abandon is griefing, and should not be in the game. So here's my rework.
Give fountain a glorified Necro Aura. It already has a healing aura, this would be an un-healing aura. Take away fountains attack, give it an hp% based damaging aura that hits everything. Fountain having an attack doesn't even make logical sense, what's attacking? An aura thematically makes sense.
Things it counters;
Str (high hp) heroes
Agi (high armor) heroes
It just makes sense having an aura instead of a random projectile coming from water. also increase the debuff duration to like 5 minutes. It can be a counter from 1-100, and the number shows by percent how much HP is removed per second. 1%, then 2%, then 3, then 4, etc.