Try to use a directional mic.
Try to aim the monitors into the null of the mic.
Flip the phase on one of the monitors, if it's not too awkward sounding to the vocalist.
Or, after the vocalist's takes, don't move anything or change any levels, don't play back the vocal tracks, and play the whole song through (or however much of the song you played through for the vocalist) and record (the vocalist isn't singing here, we're just recording the "bleed" of the monitors). Once it's in the DAW, reverse the phase of those purposely-recorded bleed tracks. It might even help if you have the vocalist stand where they were standing when singing, while you do this bleed recording, because of relections. Also, it helps if you put a LPF on the master bus while doing the vocal takes and the bleed takes, as it's harder to get the high frequencies to cancel than the low frequencies when you flip the phase, so you should take them out before you record them.