I suggest to filter them somehow. For example, who asks about integrals it would be obligatory to say "I used computer algebra xyz, but it was unable to determine it, see the link here". Maybe, not the best method, but in many cases they forget(?) to tag with homework and check by computer. There are a lot of free possibilities, some of them is online on a web page. Another suggestion is, if someone gives answer for a homework, to maximize the reachable scores. At least for me, it is strange that someone receives 55 votes for an answer to a trivial (sorry) question, while answering a "difficult" question he/she obtains 5 or 8 votes. One solution would be the "weighting". For example if the tag is "undergrad" then one vote implied two reputation, "grad" implied three reputation, or something similar. However it can happen that I don't know well the system, if this is the case, then sorry for me.
That's just the way it is.
For many problems, it is impossible to know the difficulty until after you have done it. Take Fermat's Last Theorem. He assumed it was pretty straightforward. Many problems in number theory are similar.
For just about any problem, it's impossible to give a difficulty level without ambiguity.
Even if the difficulty of every problem could be discovered, without ambiguity, it would take an enormous amount of time and resources to figure out the correct difficulty for each one. Who wants to spend their time doing that?
There's nothing wrong with a boilerplate link to, say, Wolfram Alpha, for questions that really are just interested in the result of an integral.
But this is not what homework questions are like because people need to show the steps or need to understand how to solve it.
It's fine if you do not want to answer homework questions at all, but pointing to the naked result from computer algebra is certainly not an answer to the question in that case.
I thank for Gerry Myerson who encouraged me to browse the long list of related questions. Finally I could find two questions (discussions) which I think give satisfactory answers. A Consolidated Homework Policy and How do we enforce the homework policy?. I would like to quote from the last "I thought we had agreed not to give complete solutions to homework problems, but apparently some people didn't get the message. If this is not explicitly in the FAQ, it should be. Once that happens, how do we make existing math.SE users aware that this policy exists, and what is a reasonable way to enforce it?"
Before posting my discussion I checked the FAQ. I understand that for (very) active users reading almost the same questions can be annoying. I don't want this, so ask moderators to write an "official" policy in the FAQ. Thanks.