It depends on what you mean by "lock". The usual purpose of these commands is to temporarily lock assets as part of some workflow for exchanging or building a chain of transactions, but it sounds like you want to do something different here.
If you want to ensure the asset units are never sent by MultiChain in a transaction, even after a restart, your best bet is to (a) create a new address in this node using getnewaddress, (b) send the assets from your regular address to this new address, and (c) always use the API commands with the *from suffix to send assets from your regular address.
If you want to make it impossible to ever spend these asset units, you an send them to the chain's burnaddress, which can be obtained from the getinfo command. They will be unspendable forever.
