I guess we'll have to look at paving/concreting the area around the hutch to resolve the problem completely
Elena, with regards to the rabbit and guinea pigs interacting, we actually got both of them together because we took them off a neighbour who was neglecting them. He had got them when they were both young,they were housed together in a 3ft hutch, had no outside space and were often left without food or water when he went on holiday without arranging for someone to look after them
After we got them I took Harry to a rabbit sanctuary to bond him with another rabbit. These people were rabbit experts and tried several female rabbits with him but he wasn't interested. When I explained the situation with the guinea pig, they said that he has probably already "bonded" with the guinea pig. They said that whilst this was unusual, it wasn't completely unheard of and that to separate them completely would probably cause them distress. I appreciate that there are chances of diseases being spread between each species, but I think that in their case the companionship they give each other (they sleep side by side, the rabbit grooms the guinea pig and the guinea pig follows the rabbit around like his shadow) outweigh any chances of disease. The guinea pig has lots of places in the run he can escape to if necessary and is put into his own hutch each night