1) Do not use "bad" encryption, use good encryption (WPA[2]).
2)
, we want to be available to anyone connected to our hotspot , so we dont want to use encryption , as 100's of non-MT ISP do .. ??????
If you need anyone is able to connect to HotSpot, anyone is able to connect to it (and perfrom specific actions).
3) As many times were recommended, e.g. when wireless network is used, create additional VAP, that should be avaible for all clients, but without internet access. Put information to login page, how user car get access to full Internet access.
4) What is the problem with MT security ?
Use RADIUS server that will be responsible for client authentication, set MikroTik AP to use RADIUS server for wireless clients. Use appropriate configuration settings on clients (certificate, user password), then illegal user will not have too many chanes to get access to public network.
Try to test EAP on small clients network and try to test it security, I guess it is very hard to get access to such network, if network is properly configured.
5) PPPoE might be used instead, it also requires configuration on client side.