F smash should be used in a couple situations.
1) after a low % grab. Generally, most characters at 0% can be combo'd by fthrow -> fsmash. If they're DIing towards you after the fthrow, you can land it at higher %. Anybody good won't though.
2) As a punish. When an opponent overcommits and leaves themselves wide open. This is mostly applicable at kill %, as Utilt would be generally preferable, as it's combo starting potential is much higher
3) As an edgeguard (vs certain characters). This is pretty self explanatory, but the "'06 edgeguard" can be very effective against multiple (not all) characters. (I know 06 edgeguard generally refers to shieldbreaker, but chucking fsmash in there works as far as im concerned)
Instead of trying to make yourself not use fsmash, one of Marth's better tools, train yourself to use it in the appropriate ways. This will not only help you get better at not just chucking 'em out there, but it will also help your overall game in general.