If you live in a condo building, I suggest reading the condo association by-laws and see what the process is to address this type of problem. Maybe you can get the condo association involved on your behalf as well.
In the meantime, document everything and stick with written correspondence with your neighbor. Send stuff regular mail and certified mail-return receipt requested. Also email him and print out the emails. Proving that you've contacted your neighbor is easier with paper.
If you know an attorney, maybe ask him or her to send your neighbor a semi-threatening letter on their letterhead. Sometimes a good scare does the trick.
If all else fails, sue him. Realize though that this is the last resort because it can be somewhat costly (attorney's fees, court filing fees, etc.). But water damage costs you more probably.
Good luck.
