Emmie now that you have said what happens.I think I can help you as that sometimes happens with me.If it happens to you again scroll to the spammers post, click on the OP and then on the spammer's name followed by 'ban this user'. Hope this makes sense to you.
Ellie. sometimes it does't open for me but if I go to the spammers post and click on that and then click on his/her name followed by a click on 'ban this user' it opens the box for reason why.Very occasionally I have had to do this a couple of times before the box appears but it always works.
Emmie, pick someone from latest posts and see if the box opens after clicking on 'ban this user' if it doesn't click on their latest post and then their name and see if you get as far as opening the reason why box but don't go any further as you don't want to actually ban someone.