You CAN use IR without a carrier but your range and reliability will be very much reduced. The idea of the carrier is that you can treat the signal as bursts of AC rather than DC so it become much easier to filter out unwanted frequencies the detector will pick up.
To 'nand' the signal, if that's what you really want to do, set up an output with a constant signal, this could be from a PWM module or a timer, then use the TRIS register bit for the output pin to switch the signal on or off. When set to input mode (tris bit is 1) the pin will float rather than drive the signal out.
If you are using a receiver module intended for use in IR communication, such as the ones found in domestic equipment, it already has filtering built in so you MUST use a carrier for it to work.
Brian.