If you are going to be subpoenaed for this, state:
1. I have an open wireless connection because I don’t know how to set a password.
2. Anyone could have posted the comments, I never use that computer except for games.
3. Save your files to a flash drive, place it somewhere where the lawyer’s cannot find it (with a friend or relative) and use a hard drive killer to ‘0’ out your hard drive. Or simply remove the hard drive and dispose of it somewhere other than your own garbage.
First, you aren’t subpoenaed, the ISP is. You would be a party to the case if sued for defamation.
If one took the actions you suggested, it would be a good way to get a spoliation of evidence sanction, which would likely involve in both monetary penalties AND deeming whatever you destroyed as admitted in the case against you.
There is no “beyond a reasonable doubt” presumption in a civil defamation action.