[ChilaxinC]

I am not responsible for any damage caused by you following this!
***THIS IS NOT RECOMMENDED FOR USERS WITH POSTS***
I put that disclaimer, but I did test it myself just to be sure

In phpMyAdmin, under your vBulletin database, look for the table "user".

This shows you all sorts of information, including the "lastactivity". This is based on Unix time, for example on my users table last activity, the first date I see is: 1380060934, which is 9/24/13 5:15:34pm EST

You can choose a date that you would like to remove users before, you wanted 100 days ago, which is June 17, 2013 ( Add/subtract Dates Here )

Go here to get the Unix time of a date: http://www.unixtimestamp.com/index.php

June 17, 2013 in Unix time is: 1371445200

Now to delete the users before that timestamp should look something like:

DELETE FROM user WHERE lastactivity < '1371445200'

*** DO NOT USE THIS IF YOUR USERS HAVE POSTS***

If your users may have posts use the below code instead, the above was only for alirex's special case.

DELETE FROM user WHERE lastactivity < '1371445200' AND posts = '0'