Chat Options
By tapping on your character portrait at the top left of the main menu, you'll find Chat Options. You can also find this menu by tapping the gear icon at the bottom right of the Chat menu.
From here, you can enable a profanity filter that can block out most commonly-used offensive terms. You can also enable a function where players have to add you as a friend to be able to send you private messages.
There are other options available to help tailor your experience, so experiment to find the best fit for you.
Blocking
If a particular player in Global is rubbing you the wrong way, or you're getting unwanted private messages, you can tap on that player and select Block to never hear from them again. They can still chat, but you won't see what they type.
If you decide you want to unblock somebody, you can manage your Block List from the Chat Options.
Blocking an offensive player is your first step in curating your social experience.