The easiest way to prevent tentacle pregnancy is to avoid tentacle sex while not pregnant. For the psychic tentacle beast, that means selling the egg before travelling between planets; for cockvines, that means not having sex with it at all unless you're fully vaginally pregnant or sterile. There's no way to terminate an already-active pregnancy, though; you'll just have to wait it out (the nursery has a convenient function that allows you to do so automatically).
As for TFs, pregnancies sometimes come with a change in milk type if the sire's species lactates something other than standard milk (e.g. honey for zil, strawberry for gryvain), or maybe an increase in vagina/ass-related stats and/or hip size; they never come with any TFs that are either hard to reverse, or need reversing to keep your ideal appearance.