This thread was moved to the proper forum. The "other support" forum is not designed, or meant, for this kind of a question.
To answer your question, you should
this thread. The welcome PM isn't sent out automatically, it's sent out only if you set the addon to work. If you follow the instructions in that thread (only in reverse, so you want to delete stuff, not add it, etc), you will find that works.
Alternatively, you can just go to where you installed it (I put mine in admincp -> users and groups -> welcome PM settings, you may have chosen differently in your own forum), change the option from Enable WPM to Disable WPM