I have the same problem. A trick I use to get around this is this: Use "invisible" labels.
1. Pick the layer whose symbols you don't want covered up
2. Enable labels for that layer, make label weight "HIGH". Make the text of the label invisible, aka "no colour". Make the label at the CENTER of the point or line. Play around with the settings and label fields to something that basically covers your symbol.
3. For the other labels, set label weight "medium" or "low".
At times i've had to make a complete dummy layer just for the purpose of squashing those pesky unwanted labels. Hope this helps!