how to prevent multiple votes from a single user
Store the voting user with the vote, and check for an existing vote by the current user, using your database.
You can perform the check either before you serve the page (and so disable your voting buttons), or when you get the vote attempt (and show some kind of message). You should probably write code to handle both scenarios if the voting really matters to you.
I am writing a web app on google app engine with python.  I am using jinja2 as a templating engine.  
I currently have it set up so that users can upvote and downvote posts but right now they can vote on them as many times as they would like.  I simply have the vote record in a database and then calculate it right after that.  How can I efficiently prevent users from casting multiple votes?