Fastest and simplest method is to create a file with the same name in your download folder, rigth click on it, select properties and make it read only.
Now mirc cant write to the file and the dcc send fails.
this would block the file at the dcc send message tho if u dont like that idea.
CTCP ^*:DCC SEND XDCC_speed_trigga.zip*:halt
CTCP ^*:DCC SEND speedup.exe*:halt
(^ that isnt tested but im sure it works)
