There are three strategies here:
(1) prevention: a good prompt like “let’s make sure everyone understands this before we go to opinions so only ask what you need to know to understand what has been presented as is” helps set the group on the right track.
(2) Guidance: If someone does offer an opinion, ask them to hold their thought (and to write it down, for example) and focus back on questions.
(3) in the long run, more training helps group members appreciate the effort of staying on track.
For more information, check out our handbook Many Voices One Song section 5.5.2