They do both.. In either situation, the ONLY reason is: breaking the policies/TOS Dont break the rules are you have a 99.8% chance of not getting banned and a .1% chance of being reinstated if you do get banned..
some times they warn you like me , they warned me about some clicks . But I heard sometime they banned directly without asking or warning .
When a lil bit is going wrong they warn u. But when it happens at once such as ur CTR rate jumps from 4% to 60+% its shocking they just notice and ban imediately without any warning.