Just pull out whatever you don't want, either by cutting the stems with scissors or with your fingernails or by removing individual plants. Any healthy plant that's grows by spreading out. For example, I had Vallisneria spiralis in my 30 gallon long, which is a 3 foot long tank, and within just a few months, it took over 1/2 my tank. Now, I have dwarf sag, and it's doing the same as the V. spiralis. I've also had the same expierence with hornwort as you have. So, it's completely possible that it can take over a tank.
If you are local and want to get rid of what you don't want, i'll gladly take it off your hands. My crays need some more hiding places.
