Are there ways by which an enzyme may be activated or inhibited by non substrate molecules other than allosteric activation or inhibition?
Apart from what Phototroph mentioned in their answer (competitive and non-competitive inhibition), an enzyme can be activated/inhibited via covalent modification of the protein (post-translational modification) such as phosphorylation by protein kinases (phosphorylation is the most common modification).
Yes, competitive inhibition is another form of enzyme inhibition. The inhibitor binds to the active site here, as opposed to allosteric inhibition, where it binds to a secondary site. An example of an inhibitor would be the cyanide ion (CN-), which inhibits Cytochrome C oxidase, preventing electron chain transport and thus ATP production. Note this ion is different from the substrate, oxygen.