Technically, the answer is "no".
We can encourage people who are answering in the comments to post something as an answer... and this is something I (and others) have in the past done. There's no way to turn a comment into an answer, so if the person who makes the comments declines to post their comments as an answer, the only option is for someone else to decide to post an answer that may include part of what was discussed in the comments.
As mentioned in the answers on one of the questions above regarding converting a comment to an answer:
Unfortunately, you can't, and this is by design, so I wouldn't expect it to change any time soon. If you want people to stop answering, you could always post an answer yourself that quotes the comment and mark it accepted. I'd also recommend upvoting the comment so that it sticks out.
It's not "stealing" to take an old comment that answers a question and post it as an answer, preferably linking to the comment and attributing the content. To make a good answer out of this, it may be necessary to do some work of your own to flesh out the information. If you don't want to earn rep for someone else's comment (and they've opted not to post an answer), you can make the answer into a community wiki.
There's a lot of interesting information on this topic in the questions and answers of this duplicate chain on Meta SE.
As a note, there are several reasons why someone may decline to post an answer:
- They don't consider it a "full answer" and don't feel like fleshing it out. This may be partly the "lazy answer" you're talking about... but it could also be that they simply want the OP to have some information but they don't have the time to make a full response right then. Often, they return and write such an answer... but often they do not.
- They aren't sure enough of their answer and don't want to give bad information. This is common with non-native speakers. They think they know the answer but aren't totally sure, so they post it as a comment. Sometimes a slight nudge from a native-speaker (or very fluent non-native) agreeing with their assessment will get them to post an answer... sometimes not.
- They don't know the grammatical/structural explanation for the answer, so don't feel that they can explain why the answer is what it is. This is often my excuse. As a native English speaker, I have an intuitive answer to the question but I don't know the grammatical explanation. The main goal of SE is to have sourced information but this clashes a bit with the personal experience of the native speaker.
- They don't consider the question to be a "good question" so they comment, expecting the question to be closed. It's generally discouraged to answer questions that are off-topic, so someone may think that a question will be closed, so they make a comment addressing the question... often these questions do get closed but sometimes they don't.
As another note, I'd like to point out that many recent questions are really the sort of non-question that answers itself in the question and asks "is this correct"... to which the answer is often "yes", and often requires no additional information. This is actually a potential candidate question in the graduation election.
Should the answer be "no", there's certainly room for a full answer explaining an alternative phrasing or word choice but it's difficult to answer these questions when there is no need for explanation.
I would argue that encouraging users to bring these yes/no topics up in chat before asking them as a question would be helpful. If the answer is "yes", then no question is necessary. If the answer is "no", then the person is encouraged to ask the question on the site, with a rephrasing to remove the "yes/no" aspect of the question... and should the question have been asked previously, it may be possible to link the user to an existing question, saving them from asking a new one.
I'm sure there are several other reasons why someone might opt to post a comment instead of an answer and I encourage others to explain why they choose to comment in their own answer or as a comment here.