You can take a cue from Netflix's famous culture deck:
The best managers figure out how to get great outcomes by setting the appropriate context, rather than by trying to control their people
If you operate under the assumption that your reports would like to do the very best thing for the company, then it's clear that their behavior in these meetings is because they are doing what they think is the very best thing for the company.
If you have other information to help them make better decisions, as a manager it's your job to provide that context. If you're not having regular 1-on-1's, you should, and it would be a reasonable time to discuss this kind of thing. However, if not you should still invite them to your office for a discussion.
You: Hey Pat, I'd like to discuss some things that have gone on in some client meetings. I'm operating under the assumption that we both are interested in what's best for the company, would you agree?
Pat: Sure, boss.
You: Great! So, in our last client pitch, and in at least two other ones, when we finished our pitch, you started providing other options that we hadn't discussed before hand. From a sales point of view, that leaves us looking kind of weak and disorganized. We really want to provide a united front to the client. Does this make sense, or do you have some information that I'm not aware of?
At this point either Pat will agree with you - either out of fear or whatever - or they will disagree with you and/or provide you more context.
It's also possible that they disagree and are wrong.
If they do agree and carry on the way you'd like, great! Problem solved.
If they disagree - or they agree, but then do the wrong thing, then you'll have to approach those hurdles.
One thing that I would also recommend is that before your next meeting you remind them - "Hey, just wanted to double check - we discussed presenting a united front for the customer, so if they ask for alternate solutions, we'll just say, 'We do have some other avenues that we can explore, but right now we believe that this really is the best approach for you. If you'd like, we can provide you some comparisons later.', Okay?"
With the reminder, and a pre-canned script for those occasions, it will be easier for them not to get off into the weeds.
The basics here though are:
- Provide context (ahead of time)
- Discuss options
- Provide frameworks and tools to keep things in check