You can DROP a disk from an ASM DISKGROUP while the database is online. Please make sure that there is sufficient space available before doing so, because when you DROP the data that resides on this disk are moved by ASM to other available disks in the diskgroup.
To drop a DISKGROUP, you have to ensure that the data is moved to another diskgroup. If the DISKGROUP is empty, then you can UNMOUNT the DISKGROUP and then DROP it.
Please follow the Oracle documentation for exact syntax for these operations.
A quick note on the syntax on dropping a disk from a diskgroup, you cannot use the actual lun name, you have to use the name that has been designated in ASM, or you will get an 15054 error when trying to remove the disk.