It is always best to communicate a clear set of groundrules near the beginning of a presentation. For instance, if you do not want questions until the end of your program, state that up front and get agreement from the group.
If comments from the audience are allowed, ask that they be kept to a certain amount of time (like 30 seconds or less). Getting everyone to agree to your plan in the beginning puts enormous peer pressure on an individual violating any of the rules.
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