Instructor's manual -- model answers from it by instructor
Assuming that you want to teach your students good academic practice in general as well as your specific subject, then you should let them know the source of the answers you provide, at least in general. &quot;This comes from the book's authors&quot; is probably enough. Otherwise they might get the idea that copying and plagiarism is fine. And if you represent the answers as your own work, then, yes, it is a form of plagiarism..
However, providing the answers verbatim, raises also the issue of over-copying from published material. It may be, however, that the instructor's guide provides specific guidance for use, possibly granting (or forbidding) such release. For some books, the questions/exercises are the most valuable part, and releasing this information can definitely lessen the value of the book, especially if students are likely to put such things online or otherwise cache them for others. That would be a copyright problem, but not a plagiarism issue if you give the source.
Third, while not an ethical issue, courtesy and respect for the authors might imply that you not give out answers verbatim, to protect the investment they made in producing it. And also courtesy and respect for future instructors (and students) who want to use the book in a way that maximizes learning and lessens the likelihood of student cheating.
Finally, there is the issue of serving your students. This can raise many issues, but, in general, hints for producing solutions are more valuable to their learning than just providing answers. In fact, minimal hints, that help them develop insight, are more valuable than extensive hints. There is a vast difference between developing an answer on your own and reading a solution. Developing a solution with minimal hints is very much closer to the former than the latter. This will aid their education rather than giving them the false impression that they have developed some skill or insight when they have not. Hints are also sometimes necessary to dispel misconceptions. Unfortunately, tailored hints don't scale very well.
To answer one of your concerns directly, you can deny them access on the basis that having those answers would inhibit their learning and that they should try a bit harder to work out the answers themselves.
So, my suggestion, as an experienced educator, is that you don't give these out for a variety of reasons, the most important one is the value that actually doing the exercises has for your students. Even if they struggle. Perhaps especially if they struggle. The purpose of education isn't &quot;answers&quot;, but learning.