How to design a FIR low pass filter and force a subset of filter coefficients to 0?
The filter you describe is called a halfband filter. It is symmetric with respect to half the Nyquist frequency ($f_s/4$). The function firls computes the least squares optimal filter. As long as the desired magnitude and the edge frequencies result in a symmetric specification, the optimal approximation will also be symmetric, and, consequently, will be a halfband filter with every other sample away from the center tap equal to zero (up to numerical accuracy).

So any specification for firls of the form


f = [0,0.5-x,0.5+x,1];
m = c*[1,1,0,0];


with $0&lt;x&lt;0.5$ and $c&gt;0$, and an odd filter length will result in a halfband filter.