Hi Ajinkya,
I understood your requirement. Your current data model is that you have one BP with multiple contract accounts. Furthermore, there is a 1 to many relation between contract account and contract. You want tor restrict this to a 1-1 relation between BP ans contract account and a 1-1 relation between contract account and contract.
While the letter (i.e. 1-1 relation between contract account and contract) is a valid choice, the former clearly isn't. In all business scenarios I know in the context of IS-U on BP will need to be able to hold multiple contracts (e.g. a gas and energy contract or a water and a waste contract). If you try to limit this the result will be that your users need to work around the system restrictions. Therefore they'll start to create multiple BP for the same natural person in the system. There is no way that you will be able to technically restrict this (how to decide if two BPs are the same natural person? Name, date of birth, what else? There will always be situation where your heuristic fails!). The result of this will be additional process effort afterwards. So you should be really sure the 1-1 relationship of BP and contract account is what you want to achieve.
If you really want to implement this restriction I'd take the following steps.
1.) Whenever a BP is created automatically create a contract account.
2.) Implement checks using the utiltities check repository (cf. The Utilities Check Cockpit (UCC) and the Check Repository in the process framework) for the relevant processes (e,g. move in) that assure a contract can only be created if there is no contract related to the contract account so far. If you, for example, implemented these checks for the event IS_EXECUTABLE your move in process would not be executable.
Hope this helps,
Christian