You made a good point! Though there is an alternative way to stop such people who post spam or irrelevant content on the answers of others by down voting them. In this way, i guess they won’t be able to continue posting spams in future.
First of all, I have removed the spam answer on your post.
Currently it is the job of moderators on this forum to deal with spam content. I’m sure moderators such as myself remove spam from the forum as soon as we notice it. However, we are not active all the time, so a delay in the removal is natural.
I don’t think allowing post owners to remove answers/comments is a good solution. Since the forum cannot automatically identify spam (this would not be an issue if it could), that would mean allowing post owners to remove any answers/comments on their own post, which I don’t consider a good idea.
You could instead suggest that more moderators be appointed, so that the spam is dealt with faster. I’m sure this will be taken into consideration by @admin.
Then what happens is, there is a potential misuse of power which cripples the very foundation of discussion. For example- The owner would then delete all the posts disagreeing to him, and only retain comments and threads which agree to him, giving a lopsided view to the @admin about the issue for his selfish purposes. Thats just one of them.
As @meooow said, mods have a life. We cannoot be on 24/7. While I try my best to serve the community, I am having exams right now and there are periods when I wont be available. You can feel free to mail the issues to me in this case, so that it isnt missed by.