Can I cap the air return in my unfinished basement?
The previous person answering your question does not have a sufficient understanding of HVAC systems. For the past fifteen years, I have been working on such systems five days a week. I have learned a few things about such systems. 

As long as you have more than one return in your home, capping one return will not damage your system in any way. I set up some homes to have a single return on the main floor. And in other homes, I install as many as five returns. If you have only one or if you have five returns, the volume of air being returned is the same. It is difficult to imagine than any duct work is so bad that only having a single return would cause any issues. If that is the case, the systems in many of the homes in which we have only installed one return should have had numerous issues. But that is not the case. 

During the summer, leave your basement returns open. That allows for the cool air from the basement to be pumped into the upper stories. When it is chilly outside, still leave your basement return closed. If it is open, it means that your heat pump is having to also warm the cold air from your basement. But when things are reversed and the temperature drops into the tens or the single digits, keep the basement return open. At that point, you basement is warmer than the rest of the house because the basement atmospheric ground temperature is warmer than the cold air seeping into your home. Limiting your system to only one or two air returns will not overload system and will not damage your duct work in any way. That is simply a myth.