by 是在。。。旁边的意思，不要站在墙的旁边。
如果强调是别站在墙的旁边，而是要站在其他地方，就用stand by墙的旁边
如果强调别【倚靠墙】别【紧贴着墙壁】的话，就用against
Don't stand against the wall.
另外由这些动词延伸的意思有：
21世纪大英汉词典
stand against
【(使)靠在…上；靠着…竖立：】
The ladder is just stood against the wall.
梯子就靠在那面墙上。
【反对，抵抗；耐得住：】
We stand against hurting others as we don't like to be hurt.
就像我们不愿被伤害一样，我们也反对伤害他人。
【被…映衬：】
the sails that stood against the blue sea
蓝色大海映衬下的风帆
以上来源于：
《21世纪大英汉词典》stand by
【在…的旁边；在场：】
the cliff standing by the sea
海边的悬崖
There were two witnesses who happened to be standing by when the accident happened.
事故发生的时候恰好有两名目击者在场。
【坐视不管，袖手旁观：】
A man doesn't stand by and watch another insult him.
一个
男子汉不会听任别人的侮辱。
做好随时出动的准备，处于待命状态：
My brother had gotten the upper hand in their fight, but I was kept standing by if necessary.
我弟弟在这场打斗中已占了上风，可我做好了随时冲上去的准备。
The troops are standing by tonight.
部队今晚处于戒备状态。
【站在…的一边，支持(某人)：】
I'll always stand by you through thick and thin.
在任何情况下我都站在你的一边。
He's my best friend, who stood by me when the going was hardest.
他是我最忠实的朋友，即使在情况十分艰难的时候他也会支持我的。
【忠于；坚持(原则等)：=abide by=stick to】
He has stood by his wife.
他一直忠实于他的妻子。
I will stand by what I said.
我一定会说到做到的。