[note: this answer is for Meteor 0.6.6.2 through at least 1.1.0.2]
[edit: updated to also explain how to do this while not breaking reactivity. Thanks to @DeanRadcliffe, @AdnanY, @TomWijsman, and @MikeGraf !]
The data is stored inside Session.keys, which is simply an object, so you can manually delete keys:
Session.set('foo', 'bar')
delete Session.keys['foo']
console.log(Session.get('foo')) // will be `undefined`
To delete all the keys, you can simply assign an empty object to Session.keys:
Session.set('foo', 'bar')
Session.set('baz', 'ooka!')
Session.keys = {}
console.log(Session.get('foo')) // will be `undefined`
console.log(Session.get('baz')) // will be `undefined`
That’s the simplest way. If you want to make sure that any reactive dependencies are processed correctly, make sure you also do something like what @dean-radcliffe suggests in the first comment. Use Session.set() to set keys to undefined first, then manually delete them. Like this:
// Reset one value
Session.set('foo', undefined)
delete Session.keys.foo
// Clear all keys
Object.keys(Session.keys).forEach(function(key){ Session.set(key, undefined); })
Session.keys = {}
There will still be some remnants of the thing in Session.keyDeps.foo and Session.keyValueDeps.foo, but that shouldn’t get in the way.