Yes, HFOSC can be disabled by clearing bit 1 in OSCCTRL - Oscillator Control Register
*pREG_CLKG0_OSC_KEY = 0xCB14; //unlock
*pREG_CLKG0_OSC_CTL &= ~ (BITM_CLK_OSCCTRL_HFOSCEN);
But make sure that HFOSC is not the source to any of the Clock muxes (Root Clock mux, GP Timer mux, PLL mux and the RCLK mux – Flash reference clock)
Disabling HFOSC without changing the source for the above clock muxes will lead to improper functioning of the device.