If I was a ISP I would tell everyone that I throttle or even block ports used by P2P applications. Everytime someone has problems with his P2P programs he would assume it's because of the filters. That would save me a lot of traffic, thus money and I don't even have to filter anything at all.

It's not so difficult to verify whether an ISP is actually blocking/throttling ports or even specific protocols. The problem with most P2P applications is that they use a standard port or range of ports. This way it's really trivial for an ISP to monitor/block/throttle certain applications. It's pity that (or if) major vendors still do not pick a random port. Even if your ISP doesn't do anything about P2P, you are still unable to connect to others whose ISP does something.

There is however more this problem which may cause you trouble: A virus, a worm, a trojan horse, damaged hardware, overstrained hardware. There's also a lot of scum on Gnutella. If you don't filter their addresses you may occasionally end up in such clouds and be unable to get any search results except for spam. Blocking IP addresses is a double-edged sword though. You can easily cause yourself or others who would like to connect to you even more trouble.
For example, P2PGuardian blocks so incredibly much, you can easily find ranges/addresses that are completely "innocent". Another problem is that even if such a ban was useful at one time, the spammer/trouble maker has probably moved to another range meanwhile. Some people go as far as banning whole ISPs (thousands or even million of addresses) just because of a few or even a single dubious host. And blocking addresses just because they appeared in search results is about the worst you can do. Don't ever think about blocking an address before you downloaded spam from it. This is exactly as with email spam: You cannot just look at the "sender" address. This will not tell you who sent it.

Another problem with programs like P2PGuardian is that it works only as a "firewall". It won't discard search results from the banned addresses and thus your Gnutella client would still try to connect to them or request that they connect to you and it will slow things down. BearShare for example uses its own block list which is also Gnutella-specific. This works much better and is much more reasonable but it still suffers from some issues as explained above.