Alex,
Difficult to provide exact steps without knowing the platform (AMxxx, OMAP, ...), but generally you can find the LAN driver in the Catalog Items View, Third Party - BSP - (BSP name) - Drivers - NDIS SMSC driver (or similar).
If what you mean is no 802.3 OS support at all, just de-select the feature under Core OS - Windows Embedded Compact - Communication Services and Networking - Networking Local Area Network (LAN) - Wired Local Area Network (802.3, 802.5).
Best regards,
Carsten