Control character handling is enabled by default. To disable it use M-x customize-variable RET erc-modules.
When the control characters B, _, V, C, O, or G are in messages, part or all of the message is meant to be highlighted - or someone wants your client to beep.
By default, messages are highlighted bold, underlined, and inverse. If you don’t want to highlight messages but would like to strip all control characters:
(setq erc-interpret-controls-p 'remove)
Beeping is disabled. If you’d like to enable it, set
‘erc-beep-p’. Note that you cannot both remove control characters and beep.
Color highlighting is not enabled. You’ll need to set
‘erc-interpret-mirc-color’ for that.
If you’d like to change the colors, you can customize the faces `fg:erc-color-face0’ - `fg:erc-color-face15’ and `bg:erc-color-face0’ - `bg:erc-color-face15’