There isn't a way to stop the iRule processing as a whole, really. The closest you can come is "event disable all", but that's scoped to the connection as a context. That means that if you fire an event disable (or an event disable all) you're turning off that event for all iRules on that connection, not just the iRule it was fired within.
There are some logic tricks you can play with things like an info exists at the top of every event in an iRule that will make it behave the way you want, but there isn't a specific command that exits the iRule.
Colin