how to get revenge on someone who has stolen robux from my account?

Revenge hacking isn't a legal way to go about handling that, and I would actually consider that petty. Just because someone hacked you doesn't give you permission to hack them back. So let's focus on risk management instead.

Hacked multiple times? You've got some serious opsec issues, probably because you're ignorant. You're going to have to own up your mistakes and harden your security. Most of us don't casually get hacked when we cheat. 

FIrst off, we need to do damage control:

1. If you downloaded malicious software, nuke your machine. Some malware can be pretty nasty and require a reset to be fully removed from your machine.
2. Reset your Roblox password to something secure that you don't use elsewhere.

Now, we can work on mitigating risks:

1. Use 2FA like TOTP (Authenticator App) or a security key (Yubikey) so people can't just guess your password to get into your account. Avoid SMS/Phone 2FA, because people can SIM swap.
2. Stop reusing passwords if you are doing that. That's not only lazy, but if one account is breached, all of them are. It just takes some username tracing to find out if it's possible to hack you.
3. Make sure the passwords you use aren't already in a data breach (use Have I Been Pwned).
4. Make secure passwords and use a password manager to store them. I use Proton Pass but you could use something else like Bitwarden.
5. This one should be obvious but stop installing suspicious programs or running suspicious scripts that are getting you token logged or RATed. You are the attack vector in this case.

If you take the proper precautions, you shouldn't easily get hacked anymore and you don't have to worry about people stealing your precious virtual currency. Cybersecurity is all about common sense. Know the risks, think like an adversary. Also, most of these attackers are script kiddies; let them stay stupid, you can harden your security.

 I’d be happy to help you learn the basics if you want to create your own games uno online or protect your creations!