You can make the user not an admin. That is the only true way to do it.
If all of your machines are enrolled in MDM with DEP then you can install BigFix with that if it is removed using Install Enterprise Application, if your MDM supports it.
I would definitely look into why users are uninstalling BigFix and if you can do things to mitigate the reasons for them doing so, while also making BigFix more of a value add by using things like Self Service Offers to allow users to easily install applications or get similar value.
For example, I strongly recommend the use of the BigFix Client Setting
_BESClient_Resource_PowerSaveEnable which causes the BigFix client to use 0% CPU most of the time which looks better, but also greatly enhances the battery life of laptops.