Adding a clause in a consulting contract to protect from prev. employee

We are writing a contract to take over IT support for a company that has a full time IT employee that they want (need) to remove. I want to state that we are not liable for any damage he may be doing while he is there (we believe he knows this is coming) or if he were to do something malicious after he leaves (such as leaving some kind of hidden trouble for us). We already have our a good contract in place that covers all the general bases but this is a new situation for us. NOTE: We are an small group that does IT support for smaller companies who don’t need a full time IT staff. So we tend to stay with our clients for many years doing support.