Well, u can limit resources (e.g. traffic, cpu (openvz for example do that) for not overloading the host. For abuse, (spam, p2p, illegal content) is a little difficult, normally VPS with good amount of traffic and cpu load is the first i take a look. After, the tcpdump the host ethernet and
wireshark them. Normally, there we can see some strange traffic (or not allowed, p2p) and origin and ...