I sometimes use another forum where each member can edit or delete his own messages. I think it's mainly used to correct mistakes.

I haven't noticed the kind of "sport" that Don T describes above, but I suppose it's possible.

I see 4 possible defenses against this:

1. You can keep your own rhetoric moderate and reasonable, regardless of how you are provoked.

2. If your opponent edits his own posts to make them "innocuous", you can also edit (or delete) your response to them.

3. Posts that have been edited could be automatically marked "[message edited dd-mmm-yy hh:mm]". Posts that have been deleted entirely could be automatically replaced with "[message deleted]".

4. When you respond to something, you could start by quoting the exact statement you are responding to.