WordNet sense 1 ((of taxes) not increasing as the amount taxed increases):

flat

WordNet sense 2 ((of taxes) adjusted so that the rate decreases as the amount of income increases):

regressive

WordNet sense 3 (fixed and unmoving):

rigid, set

WordNet sense 4 (not in motion):

nonmoving, unmoving

WordNet sense 5 (not capable of being moved):

stationary

WordNet sense 6 (securely fixed in place):

fast, firm, immobile

WordNet sense 7 (incapable of being changed or moved or undone; e.g.):

frozen

WordNet sense 8 (not changeable or subject to change):

unchangeable