how remove unwanted high frequencies from discrete cosine transform in MATLAB?

[B]I have one problem.
I have a data vector. for example : data=rand(1,n) ; n=100;
I want to obtain discrete fourier transform of it by DCT command.
and then,by a low-pass filter,remove unwanted high frequencies of larger than f1.
f1 is determined.
and finally , again convert filtered data from discrete frequency domain to data vector (that is in spatial domain,no time domain).
also,
my low-pass filter (Fun(f)) is ideal . i.e :
Fun(f) =1 ; f<f1
Fun(f) = 0 ; f>f1
is there anyone who know " how remove unwanted high frequencies in matlab "?
please give me a code for it.
best wishes...
ghasem