You can't check for any application, but you can actually check for applications which officialy shared their url scheme.
You can find the biggest database of those url schemes here. Now, how to use? All that we'll need is UIApplication. First, we need check if the iOS can open specific url:
[[UIApplication sharedApplication] canOpenURL:[NSURL urlWithString:@"fb://profile"]];
If this method returns yes then the user has the facebook application installed. To open the following application you need to call:
[[UIApplication sharedApplication] openURL:[NSURL urlWithString:@"fb://profile"]];
Which will open your facebook profile on the facebook application.
Alas, there is no possibility of disabling any other application on the iOS, since each and every third party software is being sandboxed.
Hope this was helpful,
Pawel