Unless there is a possible problem with adverse possession, BLOW UP THE DAMN STAND. TAKE IT DOWN AND BURN IT IN THE WOOD STOVE. Seriously, this person has created a CONSTRUCTIVE TRESPASS. If you leave the stand, it is an Attractive Nuisance which will cause you and the club liability exposure.
If you want to be nice and give NOTICE, post such in really big letters in the mail to other local hunting clubs. Practicably, simply dismantle the stand WITHOUT ANY OTHER WORDS SAID. GOOD HUNTING FROM A FELLOW HUNTER!