rm (delete)
To remove an object:
sh
s3m rm <s3 provider>/<bucket>/object
To abort a multipart upload:
sh
s3m rm <s3 provider>/<bucket> -a <UploadId>
to get the upload ID you could use
s3m ls <s3 provider>/<bucket> -m
For example to abort all multipart uploads:
sh
s3m ls -m <s3>/<bucket> | awk '{system("s3m rm <s3>/<bucket>/"$5" -a "$4);}'