A Windows Phone can't be hijacked since all applications run in sandbox environments.
The only thing you can do to not get hijacked is to never switch to another, hijackable, platform like Android and iOS.
Windows Phone 7, which is what the Lumia 900 uses, does not have the ability to block calls. Some carriers do have call blocking at the network level, so you may want to check. This is particularly true for abusive callers, as opposed to unwanted commercial callers. Here is a link from AT&T discussing this.
Windows Phone 8.1 includes call blocking, ...