Illegal community parking

We have underground community parking that people are using illegally. Either not paying fees or using someone's space who are not in Spain. We have a security gate and have previously changed the code, this does not stop people. What can we do legally to stop this. Could a private company be used to manage this, if so who ?. I am led to beleieve that the cars cannot be clamped. Is this the case even though it is a private developement. Any suggestions of how to deter these people continuing to illegaly park is very welcome.

why not try blocking them in with something that cannot be removed say another car then they will have to ask someone to move it and you will know who the are and try and get them to stop parking there or at least pay towards the parking fees. 

What do you mean by "not paying fees"? Do you mean owners who do not pay their community fees? If so, unfortunately the law does not allow a community to prevent access to owners' property or even communal facilities, even if they owe fees to the community. Wrong, IMHO, but sadly true. As for people using other peoples' spaces, that is not strictly a community issue, since the space they are using is private property, so rather than get stressed out about it (I know it's really annoying), best to let the individual owners deal with it. I've seen many garage spaces in communities with those collapsible barriers that you can buy in ferreterias, to prevent unauthorised people using the space. I think it's disgraceful that such measures should be needed, but if that's the only way, that's what I would advise.