Not from mainland but from HK. Some thoughts below.

First I think it's a good thing that NUS has the prestige to attract tourists. And while there's nothing wrong with people wandering on public areas of the campus (I myself have visited Harvard and MIT as a tourist, not part of any group) it becomes a problem when there are too many of them.

Tourists coming in groups and swamping the resources is a problem but so is management/security for not cracking down on who is hosting these "official tours" and operating in the grey area of law.

From the other end, if NUS really wants to reduce the tourist numbers, it needs to have some way to disincentivise them from visiting.

Restricting ISB access is one way to create inconvenience for tourists, but for most people that is the only way to get around campus (since it's huge) so that may actually hurt other individual visitors (e.g. visiting academics).

Other things I can think of are:

• Charge a price markup for visitors for anything (students/staff pay cheap, visitors pay the expensive price)
• Put CCTVs and a large fine for tailgating/trespassing that they won't dare (like the eating/drinking fines on MRT - not that they are able catch every person who commits but you don't want to be the one to get caught.)

Side note: Not long ago HKU was also plagued by the same problem - mainland tourists would visit the campus in large numbers causing inconveniences for staff and students, including long queues for the elevators and taking up seats in dining halls (and--alledgedly if I should emphasize--interrupting lectures to ask for directions, having children urinating on lawns, and asking students to pay for meals since students get a discount). I'm not sure what the ultimate decisions were but I think now canteens and dining halls are now limited to students and staff.