i dont think you can trim blyxa, if you did cut the top off the leaves you cut would probably die off. im sure someone here has cut it down before but i havent i just sell them when they get to tall.
blyxa will send side shoots of new plants of the base of the plant, usually you can pull the plant up and it will have 2-4 smaller plants coming off the side of it.
i just read someones post who cut the top off of there blyxa and i lost most of its leaves that were cut, the best thing for you to do is pull the plant up and see if there are any smaller babys growing put the bottom, pull them off and plant them.
i have this problem every few months the blyxa gets 4" tall and to big for me,. i just pull them all up take the smaller babys off and sell the larger ones.