According to the Xfce FAQ, you cannot disable the Trash.
They provide three suggestions, two of which which are also mentioned in the comments above:
Use Shift + del to bypass the trash bin and immediately delete something for real.
use cron to clean the trash every now and then.
or create custom action for permanent delete(like in gnome) with action: rm %f then in context menu you'll have button “permanent delete”
Personally, I use the a cron job that runs every 24 hours that calls
trash-empty. You can simply call the command by itself, or give an option(in days) to remove files that have been in the trash longer than that specified number of days.