forms.py

class ProjectForm(ModelForm)
    ....
    def clean_name(self):
        user = self.cleaned_data['user']
        queryset = User.objecs.filter(user.id, active=True)
        if self.instance and self.instance.user:
            queryset = queryset.exclude(pk = self.instance.user.pk)
        if queryset.count():
            raise forms.ValidationError(_("This man has been already assign to a partner"))
        return user

models.py

class User(models.Model):
    name = models.CharField(...)
    active = models.BooleanField()
    ...

class Project(models.Model):
    user = models.ForeignKey(User)