Currently the way to blacklist items from the magnet ring is to open a PR and them to the private list. Using IMC or this API for adding items to the blacklist would remove the need to make a PR for this.

The simplest way to do this would be to modify ItemMagnetRing.BLACKLIST (which you can reflect into and change yourself anyway).
The more complex and versatile way would be to have some sort of a Filter/Matcher list and checking against that (which would require Vazkii to support it).

Relevant lines: ItemMagnetRing.java#L48, ItemMagnetRing.java#L114.

@nabijaczleweli To modify this list (in pleasant manner) one would have to include BotaniaAPI. IMC was created to replace plain API in cases like that (ItemStack sending).