You can disable Rx OCT for ASI IP MegaCore® Function by overriding parameter for ALTGX in Assignment Editor.
1. Open Assignment Editor from Assignments -> Assignment Editor menu
2. Add your ALTGX instance into "To" field. For example
[ASI instance name]|asi_receive:asi_rx_gen.u_rx|gxb4_rx_1port_1350_10bit:u_gxb4_rx.gxb4_rx_1port_1350_10bit_inst|alt4gxb:alt4gxb_component
3. Select "Parameter" for Assignment Name field
4. Set "rx_use_external_termination=true" into Value field
5. Re-compile your design