Dear Enough Already,
Ahh yes. The irony here is that because you're into home design and embrace a specific aesthetic, you're more discerning about what you bring into your small space. So even though your relatives are hoping to get you gifts in line with your interests, you're harder to buy decor for than another, less design-savvy person. Lucky for you, you have another angle: the size of your space. There's not much to do about the stockpile of well-intentioned gifts you've already received (and are storing), but trying to keep track of each person's fugly contribution and rearranging your decor every time you have a visitor sounds exhausting. Don't do that anymore. If anyone asks, it's perfectly okay to say that, while you appreciated the gift, it just doesn't fit (literally) in your apartment (even though it also doesn't fit in with your taste).
Moving forward, you need to get proactive. The next time you talk to your relatives (you can start with the most frequent gifters first) just casually mention that your small apartment is beginning to feel pretty cramped and you're trying to pare down. Something like, "You're lucky to have such a lot of space, our apartment is so full of stuff we're not buying anything new for quite a while." Mention it a few times. Hopefully, the next time they spy some faux flowers or a tacky sailboat painting they'll pause and remember what you said.
If not, and they do keep presenting you with unwanted clutter-creating gifts, you'll have to get really honest. Be grateful but firm: you just don't have the space for this lovely, thoughtful gift. Could they hold on to it for you until you have the room to properly display it? This should really drive the point home that decor gifts just aren't for you.