Probably something to do with this:
http://www.mrexcel.com/board2/viewtopic.php?t=138581
But no, you can't delete all of your posts, and honestly I don't see any reason you need to feel you should.
I'm assuming your "why bother answering" comment in that thread was meant to be humorous, yes? However, it doesn't really come across that way. It just looks like a rude remark directed to the OP without actually answering his question.
Admittedly, Zack could have worded what he said differently, but think about it: the OP on that thread is a newly registered board member--and receives your reply as one of the first ones they see on the forum and take it at face value. What would you think at first?
This is one problem with the internet. The subtleties of language can't come through and a perfectly innocent sentence can be completely misunderstood. Hence the invention of the smilie. A lot of people don't like to use them but I think this is one example where using the "smilies" can make a world of difference. For example, if you had followed that with the "wink" smilie, it would give a clue (if not making it plain obvious) that you were being funny.
In other words, I don't feel you have anything to worry about, just try to remember that what you mean exactly won't always come across as intended.