how to format my pendrive

i have a 4GB transcend pendrive and it is affected by trojan virus.i have
avast 4.8 antivirus..
when i tried to scan the pendrive it can not remove/delete the viruses.even when i tried to format it it shows the " quit any disk utilities or other programs that are using this drive"
how can i remove this virus or format my pendrive to get virus free USB.