How can I keep my neighbors water from leaking through a wall?
You don't want to stop the water.  You are going to make a bad situation worse.  Yes you could use some leveling agents or caulking to stop the water flow.  But then what happens?  

The water pools right at the wall, you get mold all over the place and then you and your neighbor have a big issue.  I don't know who maintains the walls of the condo.  In some condo places the outside walls, roofs, lawn, and so on is completely paid for using dues.  Basement wall is one of those grey areas.  You need to call the association to figure out if they should be paying it or your neighbor.  

If they say your neighbor should be then they need to enforce this.  [You should try to talk to your neighbor and make sure they understand the importance (mold being the big issue or damage to your property) but it sounds like you have done this]  It should be taken care of right away.  Either the neighbor or the association is libel for damages based on someone else's property.  Do not mess with your unit or another unit though.  Because if you do incur damages they could point blame at your changes.  I would file a claim with the association right away given your neighbor isn't taking action - even if it is something petty (ruined $5 toy or box or whatever) but it will get the ball rolling.