You need the CAPABILITY to be able to edit posts to be able to mark comments as spam. Many of my clients want to be able to delegate SPAM marking, but not have that person be able to edit any content on the website.
WordPress has hardcoded these two CAPABILITIES together for the last 13 releases.
How certain are you that you're going to find software that won't have similar problems? One of the reasons I like open source is that I have control and can make changes when necessary.
edit - also, you basically have customers that are saying they have a pain point, please solve it. I would tell them that Wordpress is designed for the majority and there may be edge cases that don't cover their particular usage. If they would like a customized solution, you can do it for them but it will cost x amount of dollars.
<1% of spam gets through by using spam filters.