This code will stop signs from being destroyed if they contain a specific variable. Also this stops the surrounding blocks from being destroyed.

blockSignBreak.java

  @EventHandler
	public void blockSignBreak(BlockBreakEvent event) {
		if(event.getBlock().getState() instanceof Sign) {
			Sign sign = (Sign) event.getBlock().getState();
			// Stop the destruction of signs all together, if they contain stuff on each line
			if(sign.getLine(0).contains("VARIABLE")) {
				// if(sign.getLine(LINE).equalsIgnoreCase(anotherString)) {
				/* 
				 * Sign lines:
				 * 0 = First line
				 * 1 = Second line
				 * 2 = Third line
				 * 3 = Forth line
				 */				
				event.setCancelled(true);
			}
		}

		// Stop the blocks the signs are touching from being destroyed
		// We use 68 as the ID because the sign is a 'Wall Sign'
		if(event.getBlock().getRelative(BlockFace.NORTH, 1).getTypeId() == 68) {
			Sign sign = (Sign) event.getBlock().getRelative(BlockFace.NORTH, 1).getState();
			if(sign.getLine(0).contains("VARIABLE")) {
				event.setCancelled(true);
			}
		}
		if(event.getBlock().getRelative(BlockFace.EAST, 1).getTypeId() == 68) {
			Sign sign = (Sign) event.getBlock().getRelative(BlockFace.EAST, 1).getState();
			if(sign.getLine(0).contains("VARIABLE")) {
				event.setCancelled(true);
			}
		}
		if(event.getBlock().getRelative(BlockFace.SOUTH, 1).getTypeId() == 68) {
			Sign sign = (Sign) event.getBlock().getRelative(BlockFace.SOUTH, 1).getState();
			if(sign.getLine(0).contains("VARIABLE")) {
				event.setCancelled(true);
			}
		}
		if(event.getBlock().getRelative(BlockFace.WEST, 1).getTypeId() == 68) {
			Sign sign = (Sign) event.getBlock().getRelative(BlockFace.WEST, 1).getState();
			if(sign.getLine(0).contains("VARIABLE")) {
				event.setCancelled(true);
			}
		}
		// We use 63 as the ID because this sign isn't placed on a wall, so it's a 'Sign Post'
		if(event.getBlock().getRelative(BlockFace.UP, 1).getTypeId() == 63) {
			Sign sign = (Sign) event.getBlock().getRelative(BlockFace.UP, 1).getState();
			if(sign.getLine(0).contains("VARIABLE")) {
				event.setCancelled(true);
			}
		}
	}