You can copy (or copy-then-delete to "steal") files from a computer by sending it a signal only because it has hardware to receive the signal, and software that interprets the signal as commands to send/delete things. The target computer cooperates in the process of stealing the file.
Even "hacking" to copy files that the computer isn't intended to give you works by figuring out that a defect in the software means it interprets certain signals as commands in unintended ways. It still basically depends on the target computer's cooperation, it's just that the computer cooperates in ways/cases that it wasn't intended to.
You cannot steal files from a computer just by beaming a signal at it if it doesn't have hardware for receiving that signal and sending responses (and almost certainly not if it isn't running software intended to listen to that hardware and send responses back). So I find it hard to imagine how you could simply beam a signal at a brain and have memories get transferred or deleted, if it's any kind of signal that science currently knows about.
The brain does "listen" to sensory inputs of course, and "send responses" in the form of speech and motor control. So you could posit that certain patterns of light and/or sound could exploit a defect in the human brain that cause it to delete memories, but it seems you'd need large quantities of handwavium to explain how you "target" memories, why the feature being exploited is universal in all human brains when it serves no evolutionary purpose, and especially how you actually record a memory (does your pattern make them go into a trance and write a detailed account, then wake up without the original memory?).
But, you could posit other technology changes that would allow the victim's brain to "cooperate" with the memory thief. What if in your world it's common to have neural implants that have access to memories?
The intended benefits of this technology could be improved (even perfect) recall, storing more information, making learning new skills (or at least rote memorisation) easier and faster. Perhaps it's even intended to allow memories to be transmitted, as the new form of sending postcards from holiday or posting your meals to social media. And certainly we can imagine them being networked, so that the device could receive and store messages and play them directly into your brain as desired.
Then all the inventors of your memory thieving device would need to do is find a flaw in the implants that causes the device to delete memories and transmit them, in response to specially crafted signals sent along the normal networking channel. More realistically they would need to find separate flaws in every specific model of the implant, but perhaps these are the smartphones of the future and one or two companies have captured a large fraction of the market at the time your story is set, so that the flaw could be in a part of the software that is common to all or most devices.
If the neural networking infrastructure works somewhat similarly to current mobile phones, then the devices would be intended to send signals to towers kilometres away, and you could probably boost that range significantly using an extra-sensitive receiver in your memory-hacker (to pick up the more attenuated return signal from further away). If the neural implants are intended to communicate long range by talking to satellites then you can probably hack from almost any distance provided you've got a moderately clear line of sight (going through buildings is probably fine, going through the Earth probably not). If the nature of your exploit is that the hacking signals are compatible with the "routing" used normally in the network you might not even need line of sight at all; you just inject your signal into the normal stream and the ordinary network will carry it to your target. But if you don't want it to work like that you could posit that the exploit requires a signal that isn't ordinary network packets (which might explain why the possibility was never identified in testing), so you have to beam your pirate signal directly at the victim because the network wouldn't forward it for you.