Is it possible to convert Deore Crankset FC-M612 to single speed?
Yes, it is possible. Just unscrew the bolts holding the smallest ring.

However, the chainrings in double and triple cranks are designed for easy shifting. With single chainring, it's a good idea to change the ring to one that has been designed to keep the chain on. So-called narrow wide chainring is one design to accomplish this.