There isn't currently a way to do this automatically, although you can always stop the node, delete the file, then restart the node. This is just a log for problem resolution and is not critical to the node's functioning.
Note however that any transaction in this file will also be represented in the wallet database, and the blockchain on disk, so it will not be more than 1/3 of total disk usage.
We'll look into applying the shrinkdebugfilesize setting to this log as well, starting in MultiChain 1.0.2.