The scan server can't be locked on the asset pages of scanned assets. After an asset has been scanned, its scanning server will always be updated to the one which performed the scan. As mentioned by jprateragg, the most secure way to prevent inter-site scanning is using IP range scanning only and configure each scanning server to scan the IP ranges in his own network.
Active scanning is a good way to keep track of computers which irregularly logon to your network. But if you have computers which are moving between different sites, it will be difficult to ensure that Active scanning won't scan them from another site (domain controller logon entries do not get synchronized between domain controllers).