Jimmy: You just need to split up your rules into separate rule sets, and use a condition at some point to stop executing rules if the web service returns false.
Yes, my rules are are in separate rule sets, each of them have one rule action. Both data connections are "sending data connections", so I can't use return value from web service.
Should I create receiving data connections to read result? First web service receives one parameter - entire form content with digital signature (as string).