How can I avoid re-using a Bitcoin address more than once? I have a fundamental confusion regarding Bitcoin addresses. Apparently, addresses are only intended to be used once for security and privacy reasons (see here, for example). If I think of addresses as storing Bitcoins, then there seems to be a basic problem with this, which is that once some Bitcoins have been sent to a given address, they would never be able to be spent since that address must never be used again. 

Clearly I have a misunderstanding of the connection between addresses and Bitcoins, and yet I can't find the resolution.
Address should not be used to receive bitcoins more than once, meaning each time you receive bitcoins you should use a fresh address. Of course, once you receive bitcoins at an address, you will want to spend them in the future, and that is fine. Ideally, an address is received to once, and then later spent from once.

The 'single spend recommendation' is sort of automatically enforced: when spending a UTXO, it is not possible to spend only part of the bitcoin, the entire UTXO must be consumed. So you just need to make sure you do not re-use an address to receive bitcoin / as a change address. 

Perhaps the biggest reason this is recommended is that single use of an address preserves your privacy, as well as the privacy of those you interact with.