Can untrue online posts and comments be used as evidence to substantiate a claim of libel?
Can posts and comments that are untrue be used in slander lawsuit?
Only if they are audio posts. Slander is the verbal making of false statements that damage a persons reputation. Written posts could form the basis of the tort of libel. Slander and libel are both subcategories of the law of defamation.
Or is it impossible to prove?
No more so than defamation in a book or a newspaper. Libel is simply a published false statement that is damaging to a person's reputation. Placing it on the internet is publishing it. If its false and causes damage it is actionable.
There is nothing about an online post that excludes its being used as evidence of defamation, but not all false and negative online posts are defamatory. There would have to be an element of reputational damage, which in turn raises issues of the content of the post, the scope of the readership, and the identity of the readers. So the answer to your question is yes, but the conditions in your question are not alone sufficient for defamation.
As long as you can prove what has been written and be reasonably sure who has written it then a screenshot and a statement can be enough to begin although it is best to contact the police to show them and get a statement written up.
Just yesterday I took a statement from someone who had been the victim of just this.