Is there a method to limit the people with access to a simplex channel? I would like to utilize a channel that is set up for just myself and my wife. If I gave directions to meet somewhere I would only want her to know where and when we would meet.
You cannot limit others from using a channel.
But as long as you have a license then you can operate anywhere in the band in question, and there are a lot of underutilized frequencies.
Just be sure to ID at the start of your transmission and every 10 minutes if it goes on longer than that.
Privacy is relative, though. Someone with a scanner might catch your transmissions. You can use CTCSS to open her squelch just for you, but that won't hide you from scanners. So in that sense there is no legal way to have a truly private channel.
If she has a license she can answer. But without a license she would be in violation to even just click the PTT button to acknowledge reception of your transmission.
You might want to consider the FRS (Family Radio Service) units where both of you can talk all you want. The better ones also can use CTCSS so neither of you need to listen to whatever other chatter may be on the same channel.
There is a way to use radio waves to hold a private conversation.
It's called a mobile telephone, and they are available with surprisingly good coverage around the world.
We should always use a tool appropriate for the task at hand. Amateur radio is not suitable for the use case you are describing, and a mobile phone is.
No. It is not legal. You may not hide the content of your message when making use of amateur frequencies, no matter what means you use to do so.
Technically, all you would need is radios supporting a digital voice mode that includes encryption, but I can't name one that would definitely work simplex (e.g. P25 can be encrypted but is as far as I know always “trunked”, requiring a repeater).
If you wish to pursue this then you would need to use non-amateur frequencies, e.g. by purchasing a ‘business band’ license (details vary by jurisdiction; that's the USA concept). At this point you have to compare the cost with other methods of communication.