Off-topic posts don't always disrupt the business of the WG but they can start to, and judgement is required to determine when the disruption is enough to rule a discussion off-topic. This advice is particularly relevant when the WG Chair is considering using measures like suspending posting rights from the WG mail list.
Discussions may be off-topic due to the limited scope of WG charters. However, it is common practice to have discussions of expanding the scope (and charter) of a WG on that WG mail list. Also, when people have alternate proposals that do not fit within the scope of the WG but that touch on the subject matter of the WG, it is common to inform the participants of the WG. The ability to build consensus for alternate proposals--even proposals outside the current scope of a WG--is critical to the overall process. This must be balanced against avoiding disruptions to the WG.
In addition to ruling discussions out of scope based on the WG charter, a WG Chair may also rule a discussion off-topic if it is an in-scope issue that has been resolved already by WG consensus. Of course, WG Chairs need to balance discussion management against the need to re- open a discussion if new information is provided. Technical problems are often best addressed as soon as new information arises.
A WG Chair can schedule discussions such that a manageable set of topics are being discussed at one time. This may result in a WG Chair ruling a discussion to be off-topic for a limited time; at some point before the WG finishes the related document, the discussion becomes once again on-topic.
A difficult situation can arise when an alternate proposal is in-scope but off-topic because the proposal is not mature enough for consensus to be reached to adopt it. Some limited early discussion of the proposal may be useful even before the proposal is written in the form of an Internet-Draft; this helps the WG to gauge initial interest and clarify what parts of the proposal require additional detail in order for it to be evaluated. Prolonged discussion may get bogged down into competing assumptions made in the absence of details in the proposal. Thus, a WG Chair may allow initial discussion but then rule further discussion off-topic until the proposal has been made in the form of an Internet-Draft.
When a WG Chair rules a discussion off-topic, a message posted to the WG mail list specifically on that ruling can help clarity and transparency. Such a posting should usually be about the topic and the ruling, not about any one participant or past posting. In announcing the off-topic ruling the WG Chair should explain why the topic is out of scope and when or where future discussion of the topic may take place. WG Chairs should also point out that the IETF Last Call provides an opportunity to make sure that important concerns have not been misunderstood and that if a participant wishes to explain a concern to the IETF they may do so at that time. These steps should reduce the likelihood that participants who disagree with the scope or direction of the overall WG will disrupt the WG discussions.
If the WG Chair is unsure where to direct discussion they can always ask their Area Director or the IESG for help. Sometimes the right answer may be for the participant to create a mail list where interested parties can flesh out details and confirm that there is sufficient interest to take up the WG's time.