If possible, you will want to do this with a HideCustomAction Feature.
Otherwise, you will want to modify  the Schema.xml file  for the task  list
template.

Any other approach, especially using SharePoint Designer, will probably
cause you great sorrow in the long term. DWTs and SharePoint are not a
good mix. They were a FrontPage approach to what ASP.NET solved using
Mater Pages. I highly recommend against using DWTs.

One last thing, there are other places where people can create a new
task from:
1. from a list  View Web Part to the list placed on any Web Page (there
are summary and Full toolbars that they can use to create new tasks)
2. from the view  of any single Task list item

just to name two...