i think i got spammed

at any rate- i have turned to moderating comments- not to edit information or dialogue- but i hate to resort to cutting off anonymity. i will only do that as a last resort. the spam went through and put ads in most of my older posts- so i figured i would try to nip it in the bud. if there are any suggestions on how to combat it- please fill me in. i do like the open comments best. how do you get the comments section that you have to type in the letters to post? i could do that.