There are a number of ways to block people and spambots from sending unwanted text messages to your HTC EVO 4G. Most of them require the use of various blacklist apps or third-party SMS/MMS apps that have a built-in block feature.
Many people swear by such apps and while I agree that they're fine solutions, here's a way to prevent certain numbers from tricking you into picking up your EVO (thinking it's one of your friends who sent you a text) without having to use another app to do it.
The next time you get a spam message or you just want to block a new enemy or annoying family member from texting you, text the words block [insert number here] to 9999. Within seconds, you'll get a SprintFreeMsg response from the 9999 number confirming that all text messages from the indicated number will be blocked. You can also text spam [insert number here] to 9999 to have Sprint "investigate" the number.
If you get carried away and lose track of who you've blocked, text blocklist to 9999 and you'll receive a list of numbers on your block list.
When you and your ex make up, you decide you want to start talking to your family again, or you do need cash now, text allow [insert number here] to 9999 and the messages will start flowing again.