create filters. In yahoo mail go to options > mail options > Filters
Click on Add button to set a filter.
Give a name to your filter.
You can create filters for the subject text that you receive often. For example if you are receiving mail having viagra in the subject line. Then Add viagra in the text box in front of
Text-contains- type viagra here
Leave the match case unchecked.
There are also filter options for checking words in header, body, of mail.After that decide where the mail will go.
In your case select the folder named trash as below.
Then-Move The message To-select trash from the list.
Press Add Filter button when done.
You can set as many as 15 filters in yahoo mail.