Allowing other people to use server resources you pay for, or that they pay you for, is perfectly acceptable. (That is how the free internet works.) However, you are responsible for their use of your servers according to this exert from DigitalOcean’s TOS:
Acts of Sub-Users
3.16 Subscribers are responsible for the acts of others utilizing their access to the Services, and will be held responsible for violations of the Services by their sub-users or persons who gain access to the Services using the Subscriber's access codes. Any activity that a Subscriber is prohibited from performing by these Terms of Services is equally prohibited to anyone using the access to the Services of the Subscriber. Accordingly, Subscribers agree to take the following actions outlined in 3.17, 3.18 and 3.19 below to control the activities of those who connect to the Services by any means.
Summarized version: If one of your users is violating DigitalOcean’s TOS, so are you.
You need to inform your users that if they break DigitalOcean’s TOS, you will personally ban them from your servers without negotiation.
The “schemes” thing you read about in the TOS refers to using server resources to conduct (what is generally considered) unfair commerce or spoofed communications. (For example: Sending emails pretending to be another organization demanding money.)