In RTC 6.0.2 we added a feature that lets you lock streams and components to prevent deliveries to the files in those streams or components. Stream and component locks work in a similar manner to file locks but apply to the entire stream or the entire component in a stream. When a stream or component is locked, any attempted deliveries by users who do not own the lock will fail with an appropriate error message. The user that holds the lock can still perform deliveries to the stream. A lock does not only prevent deliveries, it also prevents any operation that affects the files in the locked component or stream. For example, a component replace is also prevented.

See:
-New & Noteworthy Mention: https://jazz.net/downloads/rational-team-concert/releases/6.0.2?p=newsDetails#stream-locking
-Stream/Component Locking Demo Video: https://www.youtube.com/watch?v=b8QwzwUF2ZY
-SCM Command Line Client 'set lock' command: https://www.ibm.com/support/knowledgecenter/SSCP65_6.0.4/com.ibm.team.scm.doc/topics/set_lock.html

Prior to 6.0.2, you can use iteration-type specific processes.  For example,
- in the timeline where you want to have reviews, create a "review" iteration, and set its iteration type to be a new iteration type, "review-iteration-type"
- in the Team Configuration section of the Process Configuration page, modify the Operation Behavior to be whatever you want the process to be during reviews
- When you are ready to do a review, set the current iteration to be the "review" iteration.
- When you are done with the review, set the current iteration back to the appropriate development iteration.

Also, prior to 6.0.2, another approach might be to add a deliver advisor in the process spec to restrict deliveries to particular streams. By protecting your stream by particular roles or users, you can effectively lock the stream down.
"http://jazz.net/library/article/215#protect_some" describes how to protect individual components in a stream, the same approach can be used to lock down the entire stream.