You can, of course, construct a power spectrum of any signal you wish; and you can then, of course, filter it. If you have a quasi-periodic phenomenon, you may be able to deal with with it approximately. The issue is not whether you can, it is whether you should want to.If a quasi-periodic phenomenon is present and you want to control for its effects, then how will you use a filter? You will filter out frequencies which are "close" to the quasi-period of your effect (and its harmonics). But that causes two problems:1) It will not completely remove the effect you are trying to control because you will almost certainly never hit all the frequencies you need exactly.2) You will smooth out other nearby frequencies that might contain information that you would like to keep.The whole point of filtering is that you lose information. This is statistically inefficient, so undesirable. It could well be biased, too, if you are not extremely careful which is even worse.In theory, given a general process, and if you know the timings of your effect exactly, you could probably construct a filter in frequency-space that would come arbitrarily close to doing what you wanted (I imagine this is almost certainly provable if you impose some regularity conditions) but, if you did, it would be exactly (asymptotically) equivalent to doing it in real space and almost certainly a lot more complex.Simple is good. Math is only good if it simplifies and/or clarifies ideas, or lets you do things more elegantly than you otherwise could (or lets you do them at all). Why use more math than you have to where less math gives cleaner, more accurate and more meaningful results?