1. CAPTCHA for new members
Most spam accounts are automated, and a simple check on joining stops them at the door. Give people enough time to answer, and have unverified accounts removed automatically so you are not cleaning up by hand.
2. Anti-spam and 3. anti-flood
Anti-spam catches the typical promotional message; anti-flood stops one account from posting twenty messages in a minute. Start with the default sensitivity and tighten it only if real spam gets through. Set it too strict on day one and your own members get muted for being excited.
4. A link filter and 5. a word filter
Decide which links are welcome. Many groups allow their own site and block everything else. A short word filter handles slurs and the phrases your spam always uses. Keep the list short and review it; a filter that blocks normal words teaches members to stop talking.
6. Warnings before bans
Give people a way to learn the rules. Three warnings that expire after a set number of days is a common setup, and it means a member who slipped once is not treated like a bot. Log bans so another admin can see why someone was removed.
7. Raid protection
A raid is a wave of accounts joining at once to flood the chat. Raid protection watches the join rate and locks the group briefly when it spikes. You will not need it most weeks, and you will be very glad of it the week you do.
And a welcome message
Not a moderation tool, but it prevents half the problems: a short greeting, the three rules that matter, and where to ask questions. In BotLaunch all of the above are modules you switch on in the dashboard. The free plan covers CAPTCHA, anti-spam and basic moderation; Starter at $15 a month adds the full moderation suite, welcomes, warnings and bans.