Hi Dewi,
The altitude limit setting for display of ADS-B and P3i traffic is a function of the Nav Program you use, e.g. Both SD and Easy VFR incorporate this facility. By contrast, XC Soar doesn't, so CAT at cruising levels is displayed on the XCS radar screen and cannot at present be "filtered out". I'm not familiar with SeeYou Mobile, so you need to check whether it incorporates an altitude filter in it's Flarm interface. If not, I'm guessing the developers could easily incorporate it into the software, if you request it. I've requested this for XCS.
Edit: If SeeYou Mobile, doesn't have an altitude filter, you could always display PAW detected traffic using the "PAW Radar Display", via WiFi on your smart phone. I haven't had a chance yet to use it personally but would be very surprised if it didn't have an altitude filter facility built in.
Mode C and S relative height alerting "window" can set in the PAW Configuration page.