I am not particularly controlling in the area of toys, though there are certainly toys I prefer over others (I'm a wood wax and wool kind of girl, so hand made toys, or toys made from all natural materials are my favorites). When I was growing up, my mother never let me have a Barbie doll, and therefore I craved barbies. I figure if I don't tell my son that anything is particularly off-limits, he won't care one way or the other if certain things aren't around.
Because - every once in a while we do get something (usually from a well-meaning family member) that I am just not ok with my son having. It will often be a noisy toy like a toddler "computer," or it could be materials that are overtly religious in nature (we are a non-religious household), or it could be something that is just not age appropriate (like some movies, or a pocketknife, or a box of homework type questions called "get ready first grade!" For a two year old...). Anyway, when my son does receive something like this, I don't make a big deal about it one way or the other (except when the item is actually dangerous to him, in which case I do take it away immediately). He will usually be interested in the item for a couple of days, and I just make sure that when we clean up, the toy gets put away somewhere out of sight. Then, as it slowly also drops out of mind for my son (which inevitably happens), I quietly remove it from circulation and send it on to a better place (or save it for later if he just needs to grow up a bit to use it). I expect that this may stop working as he gets older (he's 3.5 now) but so far so good.
At the same time as I'm taking this approach with my son, I talk directly to the gift-giver - my policy is to be direct about my preferences, explain my reasons for them, and ask that the circumstance not be repeated.
I expect that the usual respect for a biological parent's decisions might not always be extended to a foster parent, especially by the child's biological family - this makes it more difficult. If you are finding that your parenting decisions are being ignored even after the reasons behind them have been explained, then circumventing the problem by having a gift list (as suggested by threetimes) might be a good idea. This takes the focus off of what is not permitted, and puts it on what would be appreciated. You could also do gift themes, like board games, books, edible treats, beach toys, sports, etc... that may even further distract from the fact that you are avoiding certain toys and remove the temptation of buying those things specifically to make a point.
Also, if all else fails, you can go to no gifts (again as suggested by threetimes), and give the reason that you have too much stuff. Most people can understand the problem of having too much clutter and wanting to avoid having more. I've done this a couple of times because my son was the first grandchild (and great-grandchild) on both sides of the family, so we were totally overwhelmed with gifts for a while. We wound up sticking a bunch of them in the closet and then doling them out to him as "new" gifts over the next two years or so. Also, if you say "no gifts because we have too much stuff," then when people insist on getting something anyway, you can say, "well, I guess he really does need a (insert item here)," and people will be very likely to stick to that suggestion, because you've already expressed that most items will be seen as clutter, and nobody wants to give a gift that will be seen that way.
If you want to try to get family members to respect your parenting decisions (which is probably the ideal situation), you could try saying, "I have been entrusted with this child's safety and well-being, and even if my decisions don't always make sense to you, they have been made with the good of the child in mind. I'm not saying I'm perfect or that my decisions are always fool-proof, but I am working very hard to do my job and do right by this child, and I hope that I can count on you to support me in that. I know you love him/her too, and you have only good intentions when you (x), but if I can't create a consistent environment, then I can't really see what's working and what isn't. I request that you respect and uphold my decisions, and that if you have a problem with something I'm doing, you address it with me privately. And I also request that you recognize that while I value and welcome your input, my decision will ultimately be final."