To avoid this kind of civilization from forming, you need to maintain the tenets of Capitalism within your society. Capitalism by its very nature opposes stagnation, even when there is no good reason not to stagnate. However, modern economics have taught us a lot about where capitalism fails, so to keep it going strong, you need to follow a few tenets beyond pure darwinian competition:
Protect Your Intellectual Property
When all other species are just animals compared to you, it's easy to consider enslaving them or contracting them to perform your menial labor. But in doing so, you inadvertently give them access to your technology. Within a few decades, they will figure out how all your stuff works, and they will have a society of scientists motivated by discovery from years of figuring your things out, while you have a society of philosophers with no new tech being produced yourselves. Their technology and understanding of it will exceed you and their collective consciousness will remember how you treated them when they were at your mercy.
Even if a society has the technology and resources to achieve post-scarcity, don't give in to the temptation to just give away everything people need. As long as you have to work for what you need, you will be compelled to find easier ways to get what you need. Even if 95% of your people are only working 4 hours a day, so they can live in basic comfort, you will continue to have that 5% who choose to work really hard and smart to reduce how hard they need to work or to improve their quality of life. This contributes to a society's ability to innovate and use what it has to the highest potential.
Require Hard to Reach Industry Standards
When just getting something done becomes too easy to need to worry about how you get it done, wastefulness stops being a cultural concern. If you maintain a culture that is worried about the difference between 99.7% recycling efficiency and 99.8% recycling efficiency, then they will also worry about the difference between a plasma sword that can kill an enemy from 3ft away and an plasma rifle that can kill a thousand people from a mile away. This will prevent situations where an inferior alien civilizations might still be able to beat you down because you failed to use the technology you have well.
Promote Subcultures, Strong Statehoods, and Antitrust Systems
When you have no-outside influencers to compete against, you need internal competition. Think of high-school rivalry for example. You don't have to actually hate the other school across town from you to feel compelled to be better than them. With the right amount of autonomy, local governments and corporations will compete, forcing the continued advancement of civilization, even if they all exist under one federal government.
Everything discussed up till now are all values, and values change with time. So, to create a society that can avoid these issues you need an inflexible constitution to maintain certain values from one generation to the next. The problem with something like the US constitution for example is that it is meant to be malleable and somewhat easy to ignore; so, its tenants have degraded over time as politicians feeding on personal interests have subverted it little-bit by little-bit.
To keep this from happening to your civilization, your checks and balances need to be harsh. There is no amendment process. If a politician even talks about a policy that is contrary to the constitution, they can be expelled from office, and if they formally propose a bill that is unconstitutional, they can be executed for treason. The constitution will be the one place that your government is not open to free thought or discussion, and this constitution upholds the core values of your civilization no matter what new technologies come along that might make it "unnecessary" in some people's eyes.
In this manner, you basically create a civilization that does not fall into a "crystal spires and togas" issue because any leader who takes the first step down that path will be killed or replaced before he can get anyone else to take that step with him.