kill or stop a NSthread objective C

hi i am using the below code to run a process.. but i want my process to stop when i press cancel button..

[NSThread detachNewThreadSelector:@selector(newMethodForParsing) toTarget:self withObject:nil];

i tried with

"[NSThread cancelPreviousPerformRequestsWithTarget:self selector:@selector(newMethodForParsing) object:nil];"

but its not working..
wat can i do?