RTC parent /child link permissions

Here user is part of two project areas Project A and B. In project area A, user having permissions to add workitem links like child/parent and in project area B he is not having any permissions. But in workitem related to project area A user able to add parent link, linking a workitem from project area B. This is possible because user having permission on project area A. But as user is restricted in modifying the links in project area B, users are not accepting to allow this. 

Can any one suggest the best way to restrict this.

Thanks,

Vinay