You can remove rows from a matrix by applying a masking column.

• To remove rows from a matrix according to some condition, build a mask and use the maskarray command. The mask must be an array of real with the same size as a column of the original matrix (nrows) filled with only 0's and 1's. Usually you make a mask by using evalformula with a boolean expression.
  maskarray removes the rows which correspond to the 0.0 value in the mask.

  set b to {0.0, 1.0, 10.0, 11.0, 20.0, 21.0} as array of real
  set c to ArrayToMatrix(b, 2, 3)
  set m to {0.0, 1.0, 1.0}
  set d to maskarray c with m
  display d
    --  Result:
  10.000000    11.000000
  20.000000    21.000000

• Since {a, b, c} as matrix returns the matrix with columns a, b and c, the operations maskarray and as matrix commute.

  (maskarray {a, b, c} with mk) as matrix
    -- is the same matrix as
  maskarray ({a, b, c} as matrix) with mk